The verdict in three sentences
For a software vendor in Amsterdam, a public REST API documented with OpenAPI costs between EUR 15,000 and 40,000 excl. VAT depending on the number of exposed resources and the security level. The developer portal (interactive docs, sandbox, key management) adds EUR 5,000 to 12,000 and determines real partner adoption. When 30 partners are already asking for data access, the investment usually pays back in under 18 months if integrations reach 10 to 20% of revenue.
What a public API costs in 2026
The first item is designing the API contract. Writing the OpenAPI 3.1 specification before the code (design first) avoids breaking changes that become expensive once partners are connected. Then come implementation, authentication, rate limiting and contract testing.
| Item | Scope | 2026 range (excl. VAT) |
|---|---|---|
| OpenAPI contract design | 10 to 25 resources, versioning, error conventions | EUR 2,500 to 6,000 |
| Endpoint development | Read and write, pagination, filters | EUR 7,000 to 20,000 |
| Authentication | OAuth 2.0 client credentials, API keys, scopes | EUR 2,000 to 5,000 |
| Quotas and rate limiting | API gateway, tiers per partner plan | EUR 1,500 to 4,000 |
| Outbound webhooks | HMAC signature, replay, delivery log | EUR 1,500 to 3,500 |
| Contract tests and docs | Automated tests, examples, changelog | EUR 1,000 to 2,500 |
| API total | EUR 15,000 to 40,000 |
The low end is a read-only API over about ten resources. The high end covers write access, webhooks, several scopes and a sandbox fed with synthetic data.
Developer portal, quotas and uptime
An API without a portal is an API nobody uses. Partners expect interactive documentation generated from the spec, self-service key creation and a usage dashboard.
| Component | Budget option | Full option | 2026 cost |
|---|---|---|---|
| Interactive documentation | Static Redoc or Scalar | Custom portal with guides | EUR 1,500 to 4,000 |
| Key management | Internal admin form | Partner self-service | EUR 1,500 to 3,500 |
| Sandbox | Frozen dataset | Isolated resettable environment | EUR 1,000 to 3,000 |
| Usage dashboard | Monthly export | Real-time charts per key | EUR 1,000 to 1,500 |
| Managed API gateway | Self-hosted Kong or Tyk | Managed cloud offer | EUR 0 to 600/month |
| 99.9% monitoring | Simple external probe | Alerting, public status page | EUR 30 to 250/month |
99.9% availability allows about 8 h 45 min of downtime per year. To commit to it contractually you need at least two instances, a replicated database and a status page. Budget EUR 300 to 900 per month of infrastructure for a few million calls per month.
Quotas are set per tier: for example 10,000 calls per day for a free partner, 100,000 for a paying partner, unlimited under contract. This is also the monetization lever: a partner plan billed EUR 150 to 500 per month turns a support cost into recurring revenue.
Monetizing integrations
SaaS vendors who open their API find that a customer integrated with two or three tools churns much less. Integration revenue comes from three sources: partner subscriptions, commission on sales brought by the integration marketplace and higher retention. Targeting 10 to 20% of revenue linked to integrations after 24 months is a realistic 2026 order of magnitude for an established B2B vendor.
Mini case study
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Thomas, CTO of a fleet management software vendor in Amsterdam (EUR 2.4M revenue), gets access requests from 30 partners. Chosen budget: EUR 28,000 excl. VAT for the API (read and write, webhooks) and EUR 8,000 for the portal, i.e. EUR 36,000, plus EUR 500 per month of infrastructure.
Assumption: 12 partners move to the paid plan at EUR 250 per month, i.e. EUR 36,000 per year. Two key accounts sign thanks to the integrations, for EUR 48,000 per year. Total: EUR 84,000 per year, 3.5% of revenue in year one, with payback in about 6 months once integrations are live.
FAQ
Should the OpenAPI spec be written before coding?
Yes, design first costs EUR 2,500 to 6,000 but prevents breaking versions. It also lets you generate documentation and SDKs automatically.
REST or GraphQL for a partner API?
REST with OpenAPI remains the standard expected by 80% of B2B integrators. GraphQL mainly makes sense for internal front-end clients with highly varied query needs.
How do you secure access for 30 partners?
OAuth 2.0 client credentials with per-resource scopes, secret rotation every 90 days and per-key quotas. Add a penetration test of EUR 6,000 to 15,000 before opening.
How much does maintaining a public API cost?
Budget 15 to 20% of the initial cost per year, i.e. EUR 3,000 to 8,000, for changes, monitoring and handling deprecated versions.
How long until the API is in production?
Between 8 and 14 weeks depending on scope, including 2 weeks of contract design and a 2-week beta with 3 to 5 pilot partners.
Let's scope your project. We scope your partner API (exposed resources, security, portal, quotas) with an indicative budget and an 8 to 14 week plan. 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.
