Your ERP doesn't talk to your webshop. Your CRM has no idea what your invoicing tool is doing. Someone on your team spends two hours every day copying data from one system to another — and occasionally gets it wrong.
An API integration fixes this. It connects two systems so data flows automatically, in real time, without a human in the middle. But before you greenlight the project, you need to know: what does it actually cost, and will it pay for itself?
What is an API integration?
An API (Application Programming Interface) is a way for two software systems to exchange data. When your webshop sends a new order to your warehouse system automatically, that's an API integration at work. When your CRM updates a client record and your invoicing tool reflects the change within seconds — same thing.
The concept is simple. The execution varies wildly depending on what you're connecting, how well the systems cooperate, and what happens when something goes wrong.
What it costs: real numbers
API integration costs depend on three factors: the quality of the APIs involved, the complexity of the data mapping, and the error handling required. Here are typical ranges for Dutch businesses:
- Simple integration (two modern systems, straightforward data): €3,000 – €8,000. Example: syncing contacts between your CRM and email marketing tool. Both have well-documented REST APIs. The data maps one-to-one. A developer can build and test this in one to two weeks.
- Medium integration (data transformation, business logic): €8,000 – €20,000. Example: connecting your webshop to your ERP so orders, inventory and pricing stay in sync. The data doesn't map cleanly — product codes differ between systems, tax rules need calculation, stock levels need real-time updates. Three to six weeks of work.
- Complex integration (legacy systems, custom protocols, high reliability): €20,000 – €50,000+. Example: connecting a modern client portal to a 15-year-old on-premise system that uses SOAP, has no documentation, and goes down every other weekend. Add authentication layers, retry logic, data validation and monitoring. Six to twelve weeks, sometimes more.
The biggest cost driver isn't the happy path — it's the edge cases. What happens when one system is temporarily unavailable? When the data contains unexpected values? When someone changes a field name in one system without telling anyone? Handling these scenarios properly accounts for 40–60% of the total development time.
When an API integration pays for itself
The math is usually straightforward. Calculate the manual cost first:
- Hours per week spent on manual data transfer between systems
- Error rate — how often does manual entry cause mistakes, and what do those mistakes cost? A wrong invoice, a missed order, an inventory mismatch that causes a stockout
- Delay cost — how much does it cost when data isn't available in real time? If your sales team can't see today's inventory, they oversell. If finance gets timesheet data a week late, invoicing slips
A concrete example: if two employees spend a combined 8 hours per week on data entry between systems, that's roughly €15,000–€20,000 per year in labor costs alone. Add €5,000 per year in error correction and missed opportunities. A €12,000 integration pays for itself within seven months — and the savings compound every year after.
For one client, we built a connector between their project management tool and their invoicing system. Time spent on manual invoice preparation dropped from 6 hours per week to 20 minutes. The integration cost €9,000 and paid back within five months.
When it's not worth it
Not every data transfer problem needs an API integration:
- One-time or infrequent transfers — If you sync data between systems once a quarter, a CSV export is fine. Don't automate a process that barely happens.
- The systems will be replaced soon — Building a €15,000 integration for a system you're decommissioning in 12 months is throwing money away. Wait for the replacement and integrate from the start.
- A standard connector already exists — Before building custom, check whether a tool like Zapier, Make or n8n can handle your use case. For simple, low-volume data syncing, these platforms cost €50–€200 per month and require no development. They break down when you need complex logic, high volume or reliability guarantees — but they're worth checking first.
- The volume doesn't justify the cost — If manual entry takes 30 minutes per week, the math doesn't work for a custom integration. The threshold is usually around 4–5 hours per week of manual work before custom development makes financial sense.
What makes integrations expensive (or cheap)
The single biggest factor is API quality. A well-documented REST API with consistent data formats, proper error codes and a sandbox environment for testing can cut development time in half. A poorly documented API — or worse, no API at all — doubles or triples the cost.
Before starting a project, I always do a technical assessment of both systems. Two hours of research upfront can save weeks of surprises during development. Questions to answer early:
- Do both systems have APIs? Are they documented?
- What authentication method do they use?
- Are there rate limits that affect real-time syncing?
- Is there a test environment available?
- Who maintains the APIs — and how often do they introduce breaking changes?
If the answers to these questions are mostly "I don't know," budget for the higher end of the cost range. Unknown APIs are where timelines expand.
Build vs. buy: the integration platform question
Integration Platform as a Service (iPaaS) tools like Zapier, Make and Workato sit between fully manual and fully custom. They're worth considering when:
- Your integration is straightforward (trigger-action, limited transformation)
- Data volume is low to moderate (under 10,000 records per month)
- Downtime is tolerable — if the sync delays by an hour, nothing breaks
Go custom when:
- You need complex business logic during the data transfer
- Volume or frequency is high — real-time syncing, thousands of transactions per day
- Reliability is critical — failed syncs directly impact customers or revenue
- The integration is part of a larger custom system you're building anyway
How a typical integration project works
Most integration projects follow this structure:
- Technical assessment (2–3 days) — Evaluate both APIs, map the data model, identify edge cases and risks. This is part of the discovery phase if you're building a larger system.
- Core development (1–4 weeks) — Build the connection, data mapping and transformation logic. The happy path works first.
- Error handling and monitoring (1–2 weeks) — Build retry logic, alerting, logging and fallback behavior. This is where reliability comes from.
- Testing and go-live (3–5 days) — Test with real data in a staging environment, then switch over. Run both manual and automated processes in parallel for a week to verify everything matches.
Want to know what your integration would cost?
Tell me which systems you want to connect and what data needs to flow between them. I'll give you a realistic estimate — including whether a standard tool might be enough or whether custom development is the better path. No cost, no obligation. Get in touch.