API Integration Cost: Connecting Your Systems Without Surprises
What drives API integration cost for a UK business, from build time to usage fees, transaction charges and upkeep, and how to get a quote without surprises.
API integration cost has two parts: the one-off cost of building the connection between your systems, and the running cost of keeping it working, which can include fees charged by the software you connect to. The build price depends mostly on how much data moves, in which direction, and how many awkward cases your process has. The running cost is the part most quotes leave vague, so this guide walks through both and shows you how to get a figure with no surprises.
Key takeaways
- Price an integration on its data flows: what moves, which way, how often and what happens when it fails.
- Some platforms now charge for API use. Xero's developer pricing runs from a free Starter tier to paid tiers billed monthly in Australian dollars.
- Payment providers charge per transaction, not for API access: Stripe's standard UK card rate is 1.5% + 20p.
- Error handling, logging and retries are what separate a reliable integration from a fragile one, and they belong in the quote.
- Ask for a fixed scope and price for the first version, and agree who maintains the connection afterwards.
What are you actually paying for in an API integration?
An API is the door a piece of software leaves open so other systems can read or write its data. An integration is the code that walks through that door on your behalf: it takes an order from your website and creates an invoice in your accounts, or pulls a new lead from a form into your CRM, or tells a courier to book a collection.
The work splits into a few distinct jobs, and a good quote names each one:
- Mapping. Deciding which field in system A becomes which field in system B. A customer name is easy. A product with variants, a discount code and a part refund is not.
- Authentication. Setting up secure access, usually with keys or a sign-in flow that has to be renewed.
- The sync logic. When data moves (instantly, on a schedule or on demand), and in which direction.
- Failure handling. What happens when one side is down, a record is rejected or the same order arrives twice.
- Testing. Running real examples through a test account before anything touches live data.
- Documentation and hand-over. So that you, or another developer, can understand it later.
The first and fourth items are where budgets go wrong. Mapping looks trivial on a whiteboard and turns out to hide your business rules. Failure handling is invisible when it works, which is why cheap quotes skip it.
What drives system integration cost up or down?
We do not publish a price list for integrations, because two jobs with the same name can differ several times over in effort. These are the questions that decide where yours sits.
| Factor | Cheaper end | Dearer end |
|---|---|---|
| Direction | One way (website to accounts) | Two way, with both sides able to edit the same record |
| Timing | Nightly batch | Real time, triggered by webhooks |
| Data shape | Simple records with matching fields | Variants, bundles, tax rules, part refunds, multiple currencies |
| The other API | Well documented, with a test environment | Poor documentation, no sandbox, strict rate limits |
| Existing data | Starting fresh | Years of records to clean and migrate first |
| Off-the-shelf option | A maintained connector does the job | Nothing fits, so it is built from scratch |
Two-way sync deserves a special warning. As soon as both systems can change the same customer or the same stock figure, you need rules for who wins when they disagree. Writing and testing those rules is often more work than the connection itself.
The last row matters too. If a maintained connector already links your two systems and handles your case, using it is usually the sensible first step. Custom work makes sense when the connector cannot cope with your rules, or when several connectors are stacked on top of each other and nobody can see where an order went missing.
Third party API cost: the fees you pay after launch
Before you agree a budget, check whether API use is billed separately by any system you plan to connect. Xero, for example, now publishes tiered developer pricing, and a fee like that is easy to miss when you are focused on the build.
Usage-based API pricing: the Xero example
Xero now publishes developer pricing for apps that connect to it. Its developer pricing page lists a Starter tier at no charge with up to 5 connections, then Core at AU$35 a month (50 connections, 10 GB of monthly data egress), Plus at AU$245 a month (1,000 connections, 50 GB) and Advanced at AU$1,445 a month (10,000 connections, 250 GB), with extra egress charged at AU$2.40 per GB. Prices are shown in Australian dollars and exclude tax. Sending data into Xero (ingress) is unlimited on every tier.
What this means in practice: an integration built just for your own business, connecting to your own Xero organisation, may sit comfortably in a low tier. A product that connects to many customers' Xero accounts is a different calculation. Either way, the fee belongs to whoever owns the developer app, so your quote should say whether that is you or the agency.
Transaction fees are not API fees
Payment providers usually charge per payment rather than for the API. Stripe's UK pricing page lists standard UK cards at 1.5% + 20p, EEA cards at 2.5% + 20p and international cards at 3.15% + 20p, plus 2% where currency conversion is needed, with no setup or monthly fee on standard pricing. Building the Stripe integration is a one-off cost. The transaction fees are an ongoing cost of trading, so budget them separately.
Plan and tier limits
Check whether your current subscription plan includes API access before you commit, because the cheapest route can turn out to be the plan upgrade rather than the build. Read the current pricing page of every system involved; do not rely on what the sales team said last year.
How a custom integration quote should be built
A quote you can trust is built from the data flows, not from a headline number. Here is the order we work in, and the order we suggest you expect from anyone quoting.
- List the flows. One line per movement of data: "paid website order creates a sales invoice in Xero", "refund in the shop creates a credit note".
- Collect real examples. Ten or twenty genuine records, including the awkward ones: the part refund, the trade customer, the order with two delivery addresses.
- Check each API. Read the documentation, confirm the fields exist, note rate limits and whether there is a sandbox.
- Agree failure rules. Retry automatically, alert a person, or hold the record for review.
- Fix the scope and price. For the first version, with a list of what is out of scope.
- Agree the running costs. API tiers, hosting, monitoring and who pays each.
This is how we price integrations inside our custom web app projects: a fixed scope and fixed price for version one, with an integration stage where the app is connected to accounts, payments and existing systems and data is migrated in. Later changes are priced as short sprints, or the work can pause. If you are writing the brief yourself, our guide to a software statement of work shows what to put in it.
Common integrations for UK small businesses
Most small-business integrations fall into a handful of groups. Each has its own typical trap.
| Integration | Typical flow | Watch out for |
|---|---|---|
| Website or shop to accounting (Xero, QuickBooks) | Orders become invoices, payments are matched | VAT rates, part refunds, rounding and API tier fees |
| Forms to CRM | Enquiries become leads with a source | Duplicates when the same person enquires twice |
| Shop to payments (Stripe) | Card payments and refunds | Webhooks that must be checked so payments are not missed |
| Orders to couriers | Labels and tracking numbers | Address formats and parcel sizes |
| Calendars and email (Google, Microsoft) | Bookings appear in staff calendars | Sign-in permissions that expire |
If your website runs on WordPress, much of this sits on top of the WordPress REST API and its hooks. Our explainer on the WordPress REST API covers what it can and cannot do, and WordPress CRM integration goes deeper on the forms-to-CRM case.
Build, connector or no-code tool?
There are three broad routes, and the cheapest at the start is not always the cheapest over three years.
- A maintained connector or plugin. Lowest upfront cost and someone else keeps it current. The limit is that it does what it does; if your process differs, you change your process.
- A no-code automation tool. Quick to set up for simple flows, and fine for many small jobs. The costs are a monthly subscription, often priced on volume, and logic that becomes hard to follow once it grows to dozens of steps. Our comparison of no-code vs custom development covers where the line usually falls.
- A custom integration. Higher upfront cost, and it fits your rules exactly. You should own the code, and it should come with documentation so it does not depend on one person.
The deciding question is usually how often the flow breaks or needs a person to fix it today. If staff spend time each week re-keying orders, correcting stock or chasing missing invoices, that time is the number to compare the build cost against. Our post on signs you have outgrown spreadsheets helps you put a figure on it.
Keeping running costs predictable
An integration is not finished on launch day. The software on either side will change: new API versions, retired fields, new sign-in rules. Budget for that rather than hoping it will not happen.
- Monitoring and alerts. A failed sync should tell someone, not sit silently in a log.
- A record of every transfer. So you can answer "did this order reach the accounts?" in seconds.
- Hosting. Scheduled jobs and webhook receivers need somewhere reliable to run.
- Version changes. Ask how you will hear about changes to each API and who updates the code.
- Ownership. The code, the API keys and the developer accounts should be in your name, or handed over in writing.
On our projects, hosting, backups and security updates are covered by a monthly care plan, and the client owns the code and data once the final invoice is paid.
Frequently asked questions
Is API access free with most business software?
Do not assume it is. Check each tool's plan terms for API access, and whether usage is charged separately. Xero, for example, publishes developer tiers from a free Starter level to paid monthly tiers based on connections and data egress. Check the current pricing page of each system before you agree a budget, and note who will hold the account.
How long does an API integration take to build?
A simple one-way flow between two well-documented systems can be quick. Two-way sync, messy historical data or an API without a test environment adds time. Inside our custom web app projects, integration is a planned stage after the core build, with the app connected to accounts, payments and existing systems and data migrated in before a pilot.
What happens if one of the connected systems changes its API?
The integration may stop working or start rejecting records until the code is updated. Good integrations log every transfer and alert someone when a sync fails, so the problem is spotted the same day. Agree in advance who watches for API changes and how updates are paid for, whether under a care plan or as separate work.
Can you connect a system that has no API?
Sometimes, by other routes such as scheduled file exports and imports, or by building a small API in front of an older system. These are usually less reliable and need more checking than a proper API. If a system you depend on has no API at all, that is worth weighing when you next review whether to keep it.
Who should own the API keys and developer accounts?
You should. Keys, developer app registrations and any paid API tiers should be in your business's name, or transferred to you at hand-over, with the details written down. If an agency holds them and you part company, you can lose access to your own integration. Make ownership a line in the contract, not an afterthought.
Get a fixed price for your integration
Start by writing down the flows you want, one line each, and gather a handful of real examples, including the awkward ones. That is enough for a useful first conversation. We will check the APIs involved, tell you about any usage fees on either side, and give you a fixed scope and price for the first version. Start a project or get in touch to talk it through.
