The verdict in three sentences
A production-grade custom REST API costs 20,000 to 60,000 EUR in London in 2026, depending on the number of endpoints and the security level. The budget shifts as soon as you add OAuth2, rate limiting, versioning and an OpenAPI spec usable by external partners. Plan for 2 to 5 months of development and 300 to 1,200 EUR/month of run and observability.
Cost by technical building block
A partner API is not just a set of endpoints: it is a contract, security, docs and monitoring. 2026 order of magnitude:
| Block | Standard REST | Partner grade (public) |
|---|---|---|
| Design and OpenAPI contract | 3,000 - 6,000 EUR | 6,000 - 10,000 EUR |
| Business endpoints | 8,000 - 18,000 EUR | 15,000 - 28,000 EUR |
| Auth (OAuth2 / API keys) | 3,000 - 6,000 EUR | 6,000 - 12,000 EUR |
| Rate limiting and quotas | 2,000 - 4,000 EUR | 4,000 - 8,000 EUR |
| Versioning and backward compat | 2,000 - 4,000 EUR | 4,000 - 7,000 EUR |
| Total | 18,000 - 38,000 EUR | 35,000 - 65,000 EUR |
GraphQL instead of REST adds 15 to 25 % for the resolver layer and protection against costly queries, but cuts client round-trips.
Cost per endpoint and monetisation
The number and complexity of endpoints drive the bill. A simple GET is cheap, a transactional endpoint with side effects is expensive. 2026 estimate:
| Endpoint type | Complexity | Indicative cost |
|---|---|---|
| Simple read (GET list) | Low | 800 - 1,500 EUR |
| Filtered read / pagination | Medium | 1,500 - 3,000 EUR |
| Create / update (POST/PUT) | Medium | 2,000 - 4,000 EUR |
| Transactional (payment, stock) | High | 4,000 - 8,000 EUR |
| Outbound webhook | Medium | 2,500 - 5,000 EUR |
| Batch / bulk export | High | 3,500 - 7,000 EUR |
To monetise (usage-based), add a metering and per-call billing module: 5,000 to 12,000 EUR of extra dev, but it turns the API into a revenue stream (e.g. 0.01 to 0.05 EUR per call depending on value).
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Mini case study
Thomas, CTO of a London scale-up (logistics data), wants to expose data to 15 partners. Scope: 9 endpoints (6 reads, 3 writes), OAuth2, rate limiting, OpenAPI docs, v1 versioning. Quote: contract + docs 8,000 EUR + 9 endpoints 24,000 EUR + auth 8,000 EUR + rate limiting 5,000 EUR = 45,000 EUR, plus 700 EUR/month observability. Billing usage at 0.02 EUR/call on 2M calls/month, the API generates 40,000 EUR/month: paid back in under 6 weeks.
FAQ
REST or GraphQL? REST remains the partner standard (simple, cacheable). GraphQL costs 15 to 25 % more but suits clients with highly variable data needs. Choose by your consumers, not by fashion.
Is OpenAPI docs really necessary? Yes. An OpenAPI (Swagger) spec cuts integration support by 30 to 50 % and lets you auto-generate client SDKs. It is the first deliverable any serious partner demands.
How do we secure access? OAuth2 for partners, API keys for machine use, plus rate limiting (e.g. 1,000 requests/min). This foundation is 9,000 to 20,000 EUR but prevents leaks and abuse.
How much does observability cost? Plan for 300 to 1,200 EUR/month for logs, traces, alerts and a usage dashboard. It is what lets you bill per call and spot a partner saturating the API.
Can it be delivered in phases? Yes: an MVP of 3 to 4 endpoints in 6 to 8 weeks (18,000 to 25,000 EUR), then extension. This is the recommended approach to validate adoption before investing more.
Let's scope your project. Tell us the number of endpoints, your consumers (internal, partners, public), any monetisation need and your indicative budget. 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.
