The verdict in three sentences
Building a custom API costs between 12,000 and 28,000 EUR in 2026 for a simple documented API, and 35,000 to 80,000 EUR for a robust one (OAuth2, rate limiting, versioning, developer portal). Cost does not stop at go-live: monitoring 200 to 1,200 EUR/month and 15-20 %/year maintenance are part of the budget. Price mainly depends on the number of endpoints, security level and SLA promised to your consumers.
Grid by robustness
An internal API linking two systems does not have the same requirements as a public API consumed by paying partners.
| Level | What is included | 2026 range (EUR) | Timeline |
|---|---|---|---|
| Simple | Documented REST, 5-10 endpoints, key auth | 12,000 - 20,000 | 6-8 wks |
| Standard | + OAuth2 + pagination + tests + CI/CD | 20,000 - 35,000 | 8-10 wks |
| Robust | + rate limiting + versioning + dev portal | 35,000 - 55,000 | 10-12 wks |
| Platform | + GraphQL + SLA + high availability + sandbox | 55,000 - 80,000 | 12-14 wks |
What weighs in the quote
Beyond endpoint code, four items structure the budget and are often underestimated.
| Item | Indicative 2026 cost | Why it is critical |
|---|---|---|
| Docs + developer portal | 4,000 - 12,000 EUR (upfront) | No docs, zero partner adoption |
| Monitoring + alerting | 200 - 1,200 EUR/month | Detect outages and abuse |
| Security (OAuth2, rate limiting) | 15-25 % of project | Protect data and quotas |
| Annual maintenance | 15-20 % of project/year | Versioning and evolutions |
The number of endpoints and the SLA are the two most sensitive variables: going from 10 to 40 endpoints, or from best-effort to a guaranteed 99.9 % uptime, can double the run budget.
Mini case study
Marc, CTO of a 30-person software vendor in Rennes, wants to expose a partner API so 12 integrators consume his product data and trigger orders. He picks the Robust level (OAuth2, rate limiting, versioning, dev portal): quote 48,000 EUR, 11-week timeline. Monitoring: 600 EUR/month. Maintenance at 18 %: 8,640 EUR/year.
Before, each new integrator required 3-4 days of ad hoc development and email support: about 12,000 EUR of hidden cost per partner. With the self-service portal, onboarding drops to half a day. Across 12 integrators, the saving is on the order of 130,000 EUR over two years, and the API opens a revenue channel (usage-based billing) impossible before. Payback in under 12 months.
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FAQ
REST or GraphQL for a custom API?
REST covers 80 % of needs and is cheaper to build and document. GraphQL is justified when your consumers have very varied query needs (mobile, rich front-ends) and pushes the project into platform level (55,000-80,000 EUR).
Is a developer portal really necessary?
As soon as third parties consume your API, yes. Without clear docs and a self-service portal, every partner generates costly manual support. Budget 4,000 to 12,000 EUR upfront for a portal: it pays back fast on onboarding.
How much does running an API in production cost?
Monitoring and alerting: 200 to 1,200 EUR/month depending on traffic and SLA. Maintenance: 15-20 % of project cost per year for versioning, security and evolutions. It is a recurring budget to provision from the start.
How does the SLA impact price?
Best-effort costs little; a guaranteed 99.9 % uptime demands redundancy, high availability and on-call, which can double the run budget and add contractual penalties. We only promise a strong SLA if your business model justifies it.
Can we ship a v1 then harden later?
Yes, that is the recommended approach. Deliver a standard documented API (20,000-35,000 EUR), observe real usage, then add rate limiting, versioning and a portal based on adoption. This avoids over-investing before you have consumers.
Let's scope your project. Tell us the number of endpoints, your consumers (internal or partners) and the target SLA, and we will frame the level (simple to platform) plus an 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.
