The verdict in three sentences
A well-designed public API opens a partner channel and can become a revenue line in its own right. In 2026 in Berlin, budget 30,000 to 70,000 EUR and 3 to 5 months for a documented REST API, secured with OAuth, with rate limiting and a developer portal. The hidden cost is not the endpoint code but the docs, sandbox and versioning: they decide whether your API gets adopted or abandoned.
Where a public API budget goes
Unlike an internal integration, a public API must be reliable, documented and backward-compatible for third parties you do not control. Here is the typical 2026 breakdown.
| Item | 2026 cost (EUR) | Share of project |
|---|---|---|
| Design + REST endpoints | 12,000 - 25,000 | 35 % |
| OAuth 2.0 auth + keys/scopes | 4,000 - 9,000 | 12 % |
| Rate limiting + quotas | 3,000 - 6,000 | 9 % |
| Documentation (OpenAPI) | 4,000 - 8,000 | 12 % |
| Sandbox + test environment | 4,000 - 9,000 | 12 % |
| Developer portal | 6,000 - 12,000 | 18 % |
| Observability + logs | 2,000 - 5,000 | 8 % |
The sum gives the 30,000 to 70,000 EUR range depending on endpoint count and portal maturity.
Versioning, SLA and monetisation
A public API is a contract. Breaking backward compatibility scares partners away; guaranteeing uptime costs money; and the billing model determines your revenue.
| Dimension | Option A | Option B |
|---|---|---|
| Versioning | URL (/v1, /v2) | Header (Accept-Version) |
| Target SLA | 99.5 % (standard) | 99.9 % (premium, +40 % infra) |
| Deprecation support | 6 months notice | 12 months notice |
| Monetisation | Per call (0.001-0.05 EUR/call) | Subscription (49-499 EUR/mo/tier) |
| Free quota | 1,000 calls/month | 10,000 calls/month |
Per-call maximises revenue at high volume; tiered subscription is more predictable and easier to sell in B2B. Many vendors combine both: fixed tier plus per-call overage.
Mini case study
Thomas, CTO of a 20-person SaaS vendor in Berlin, wanted to open his management software to partner integrators. We delivered a 24-endpoint REST API, OAuth, a sandbox and a developer portal for 48,000 EUR in 4 months. With a 149 EUR/month/partner subscription, 12 partners signed in year one generate 21,456 EUR of revenue, and the API becomes a sales argument that accelerates the core product. The project pays back in 27 months on API revenue alone, immediately on commercial value.
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FAQ
How long to ship a public API?
3 to 5 months for a documented API with a portal. A minimal core (endpoints + OAuth + docs) can ship in 8 to 10 weeks.
Do I need a developer portal from day one?
Not necessarily, but OpenAPI docs and a sandbox are essential. The full portal (self-service keys, quotas, billing) can come in phase 2.
How do I manage versions without breaking partners?
By versioning explicitly (/v1, /v2) and announcing any deprecation with 6 to 12 months notice. You keep the old version running in parallel during the transition.
Per call or subscription, which to choose?
Tiered subscription is more predictable and easier to sell. Per-call earns more at high volume. A tier-plus-overage mix covers both.
What SLA should I offer?
99.5 % is enough for most B2B cases. Moving to 99.9 % adds redundancy and roughly 40 % infrastructure cost; reserve it for critical partners.
Let's scope your project. Tell us the number of endpoints envisaged, your target partners and your monetisation model, and we will price the API and portal. 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.
