The verdict in three sentences
A custom REST API, documented and secured, costs between EUR 12,000 and 35,000 in Amsterdam in 2026. Price depends on the number of endpoints, the security level (OAuth 2.0, scopes, rate limiting) and whether a developer portal is included. Exposing your data cleanly is not a technical cost: it is a partner revenue channel that turns a database into a product.
What drives a REST API's price
An API is not just a list of URLs. The budget splits between contract design (OpenAPI), security, performance and documentation. Here is the 2026 breakdown for a medium API (15-25 endpoints).
| Item | Content | Budget share | Indicative cost |
|---|---|---|---|
| Design & contract | Data model, OpenAPI, versioning | 15% | EUR 2,000 - 5,000 |
| Endpoints & logic | 15-25 endpoints, pagination, filters | 40% | EUR 6,000 - 14,000 |
| Security | OAuth 2.0, scopes, API keys, rate limiting | 20% | EUR 3,000 - 7,000 |
| Monitoring & logs | Metrics, alerts, traces | 10% | EUR 1,500 - 3,500 |
| Docs & portal | Interactive docs, examples, sandbox | 15% | EUR 2,500 - 6,000 |
A full developer portal (self-service signup, key generation, quotas) is an extra EUR 6,000 to 12,000.
Security and robustness: the 2026 tiers
The security level radically changes budget and duration. The table sets out three common tiers for a B2B vendor.
| Tier | Authentication | Rate limiting | Versioning | Additional cost |
|---|---|---|---|---|
| Basic | Static API key | 100 req/min global | None | +EUR 0 |
| Standard | OAuth 2.0 client credentials | Per key, 1,000 req/min | v1/v2 in URL | +EUR 3,000 - 5,000 |
| Advanced | OAuth + scopes + JWT | Per plan (tiers) + burst | Semantic + deprecations | +EUR 6,000 - 10,000 |
The standard tier covers 80% of needs for a vendor opening its API to around ten partners.
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Mini case study
Julien, CTO of a logistics SaaS vendor in Amsterdam (12 people). Partners demanded programmatic access to delivery statuses; he lost ~8 hours/week sending manual CSV exports. Kolonell delivered an 18-endpoint REST API with OAuth and a developer portal at EUR 26,000 in 9 weeks. Result: 6 partners integrated in 3 months, 8 hours/week freed (~EUR 13,000/year at loaded cost) and a new "API Access" offer billed at EUR 150/month/partner, i.e. EUR 10,800/year of recurring revenue in year one.
FAQ
How long for a documented REST API? Expect 6 to 12 weeks: 1-2 weeks design (OpenAPI contract), 4-7 weeks development, 1-2 weeks docs and acceptance. A full developer portal adds 2-3 weeks.
Is OAuth 2.0 mandatory? Not technically, but as soon as you open to third parties, yes. A static API key is fine internally; for partners, OAuth with scopes avoids over-exposure and costs EUR 3,000 to 7,000 more.
How do I prevent abuse and overload? Rate limiting (e.g. 1,000 requests/min per key) and per-plan quotas protect the infrastructure. Add caching and pagination to hold load without multiplying servers.
Should I version from the start? Yes, at minimum a /v1 prefix. Breaking a production API without versioning can cut partners off overnight; a 6-12 month deprecation policy is the 2026 norm.
What does maintenance cost? About 15 to 25% of the initial cost per year, i.e. EUR 2,000 to 8,000, covering monitoring, fixes and minor endpoint changes.
Let's scope your project. Tell us the target endpoint count, your intended partners and whether you want a self-service portal: we quote between EUR 12,000 and 35,000. 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.
