The verdict in three sentences
Building a custom REST API exposed to partners costs in 2026 from 15,000 EUR (simple API, a few endpoints) to 40,000 EUR (rich API, versioning, SLA). A public API is not just an endpoint: it requires OpenAPI documentation, OAuth2 authentication, rate limiting, and versioning. Budget 6 to 12 weeks of development and a service-level agreement for production.
The components that drive the price
Many software vendors underestimate the cost of an API meant for third parties. An internal API can be rough; a partner API must be robust, documented, and secure, because it becomes a technical contract. Each component has its own cost.
| Component | Role | 2026 cost (EUR) |
|---|---|---|
| Endpoint + model design | REST design, resources, verbs | 3,000 - 6,000 |
| OAuth2 / API key auth | Access security | 2,500 - 5,000 |
| Rate limiting + quotas | Protection and fairness | 1,500 - 3,500 |
| OpenAPI docs + portal | Partner adoption | 2,000 - 5,000 |
| Versioning + backward compat | Evolvability | 2,000 - 4,500 |
| Tests + monitoring + logs | Reliability | 3,000 - 7,000 |
| Deployment + production SLA | Availability | 1,000 - 9,000 |
Three API tiers and their budgets
Depending on ambition, three profiles emerge. The table below gives 2026 orders of magnitude for Singapore.
| API profile | Endpoints | Key features | Timeline | Budget (EUR) |
|---|---|---|---|---|
| Essential API | 5-10 | API keys, basic docs | 6 weeks | 15,000 - 20,000 |
| Partner API | 10-25 | OAuth2, rate limit, OpenAPI | 8-10 weeks | 22,000 - 32,000 |
| Product API | 25+ | Versioning, SLA, dev portal | 10-12 weeks | 33,000 - 40,000 |
The SLA deserves attention: 99.9% availability implies redundancy, alerting, and on-call, which explains the deployment cost gap in the first table.
Mini case study
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Mathieu, CTO of a logistics SaaS vendor in Singapore, wants to open an API to his 12 carrier partners to exchange orders and delivery statuses. He picks a partner API: 18 endpoints, OAuth2, rate limiting at 600 requests/minute, OpenAPI documentation, and a developer portal. Budget 28,000 EUR, 9-week timeline.
Previously, each partner integration was custom: about 5 dev days at 600 EUR/day, i.e. 3,000 EUR per partner, plus support. With a standardized, self-documented API, a partner integrates on its own. Across 12 partners, the integration saving reaches 36,000 EUR, above the cost of the API, and every new partner becomes almost free to onboard.
FAQ
What is the floor price of a REST API? An essential API of 5 to 10 endpoints with API keys and basic documentation starts around 15,000 EUR in 2026. It fits a first partner or a shared internal use.
Is OAuth2 mandatory? Not technically, but as soon as an API is exposed to third parties, OAuth2 or a managed key system becomes essential. Budget 2,500 to 5,000 EUR for this piece.
Why pay for versioning? Versioning (v1, v2) lets the API evolve without breaking existing integrations. It represents 2,000 to 4,500 EUR and avoids costly breakages for partners.
How much does a 99.9% SLA cost? Redundant infrastructure, alerting, and on-call add 4,000 to 9,000 EUR to deployment, plus a recurring hosting and monitoring cost.
What timeline for a partner API? Budget 8 to 10 weeks for an API of 10 to 25 endpoints with OAuth2, rate limiting, and full OpenAPI documentation.
Let's scope your project. Tell us the number of endpoints, your authentication needs, and your SLA requirement: we will price your REST API, with an indicative budget of 15,000 to 40,000 EUR. 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.
