The verdict in three sentences
A custom REST or GraphQL API is budgeted between 15,000 and 45,000 EUR in Amsterdam in 2026, depending on the number of endpoints and business complexity. OAuth2 authentication, OpenAPI documentation with a sandbox and monitoring are not optional: they account for 30 to 40 % of the budget and determine partner adoption. Plan 6 to 12 weeks of development, plus a recurring cost of 1,500 to 5,000 EUR/year for the gateway, hosting and versioning.
What each API building block costs in 2026
An API is not just a set of URLs: it is a product with a contract, security and documentation. Here are 2026 orders of magnitude for a mid-sized Amsterdam project.
| Building block | Scope | 2026 price (EUR) |
|---|---|---|
| REST API 10-20 endpoints | CRUD + standard business logic | 15,000 - 25,000 |
| GraphQL or complex API | Typed schema, resolvers, pagination | 25,000 - 45,000 |
| OAuth2 / API keys auth | Tokens, scopes, rotation | 3,000 - 6,000 |
| OpenAPI docs + sandbox | Dev portal, examples, live trials | 2,000 - 5,000 |
| Rate limiting + monitoring | Quotas, alerts, dashboards | 2,500 |
| Outbound webhooks | Event notifications | 2,000 - 4,000 |
One tip: documentation and the sandbox are not cosmetic. A well-documented API cuts partner integration time by 40 to 60 %, and therefore your support costs.
The recurring costs people forget
Development is a one-off cost; operation is a permanent one. Many IT leaders discover the bill only after go-live.
| Recurring item | 2026 order of magnitude | Frequency |
|---|---|---|
| Managed gateway (Kong, AWS API GW) | 40 - 300 EUR | monthly |
| Hosting + database | 80 - 400 EUR | monthly |
| Monitoring / observability | 50 - 200 EUR | monthly |
| Versioning + maintenance | 250 - 800 EUR | monthly |
| Certificate / secret renewal | included to 500 EUR | yearly |
Over a year, running a serious partner API lands around 5,000 to 18,000 EUR. That is the price of availability and backward compatibility.
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Mini case study
Thomas, CTO of a logistics scale-up in Amsterdam, must expose delivery statuses to three enterprise clients and to his mobile app. Scope: 14 REST endpoints, OAuth2, OpenAPI docs, rate limiting. Quote: 22,000 EUR of development over 9 weeks, plus 280 EUR/month of operation. Each client integration used to take 5 person-days in CSV-file mode; with the documented API it drops to 1.5 days. Across 3 clients a year, Thomas saves 10.5 person-days, roughly 6,300 EUR of internal cost per year, and secures the enterprise contract that required real-time access.
FAQ
REST or GraphQL, which should I pick? REST remains the standard for varied partners and covers 80 % of needs for 15,000-25,000 EUR. GraphQL shines when clients have very different data needs, but adds 30 to 40 % of cost and complexity.
How long before a usable first version? An API of 10 to 15 endpoints with authentication reaches a workable first milestone in 6 to 8 weeks. The sandbox and monitoring follow over 2 to 4 additional weeks.
Do I need an API gateway from the start? Yes as soon as you expose externally: it centralises authentication, rate limiting and logs for 40 to 300 EUR/month, far cheaper than re-coding those layers.
How do I avoid breaking partners on every change? Versioning (v1, v2) and a documented deprecation policy are mandatory. Budget 250 to 800 EUR/month of maintenance to preserve backward compatibility.
Who owns the code? At Kolonell, you fully own the source code, schema and documentation on delivery, with no vendor lock-in.
Let's scope your project. Tell us your target endpoints, your consumers (partners, mobile, internal) and your desired timeline, and we will price both development and operation. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.
Mohamed Bah
Fondateur, Kolonell
Passionate about digital and entrepreneurship in Africa, Mohamed has been helping Sénégalese businesses with their digital transformation since 2020. Founder of Kolonell, he believes every SME deserves a professional and accessible online présence.
