The verdict in three sentences
For a New York HR SaaS vendor with 300 customers whose enterprise accounts demand integrations, opening a clean public API costs EUR 30,000 to 60,000 (about USD 32,000 to 65,000), and the developer portal adds EUR 8,000 to 15,000. The non-negotiable foundation: a documented OpenAPI specification, keys and quotas, webhooks, a sandbox and OAuth 2.0 security. Well designed, the API halves integration support costs and unlocks enterprise deals that would otherwise go to a competitor.
The scope of a serious public API and its cost
A public API is not your internal API exposed as is. It needs a stable, versioned, documented contract, and safeguards so that one badly coded client cannot take down your production.
| Building block (2026 order of magnitude) | Purpose | Indicative cost |
|---|---|---|
| OpenAPI 3.1 contract design | Resources, naming, pagination, errors, versioning | EUR 4,000 to 8,000 |
| OAuth 2.0 authentication and API keys | Delegated access, scopes, secret rotation | EUR 5,000 to 9,000 |
| Public endpoints (employees, leave, contracts, payroll) | 20 to 40 endpoints, contract tests | EUR 12,000 to 25,000 |
| Quotas and rate limiting | Per customer and plan, standard headers | EUR 2,000 to 4,000 |
| Signed webhooks with retries | Business events, delivery log | EUR 4,000 to 8,000 |
| Sandbox with fake data | Risk-free testing for integrators | EUR 3,000 to 6,000 |
| API foundation total | EUR 30,000 to 60,000 | |
| Developer portal (optional) | Interactive docs, console, self-service keys | EUR 8,000 to 15,000 |
HR data is sensitive personal data under privacy laws such as GDPR and US state regulations: OAuth scopes must be fine-grained (reading leave without access to salaries, for example) and every call logged.
The developer portal: what it changes
Without a portal, each new integration goes through email threads, an outdated PDF and a developer creating keys by hand. With a portal, the integrator signs up, reads interactive documentation, tests in the sandbox and gets keys without involving you.
| Indicator (2026 estimate) | No public API | API without portal | API with portal |
|---|---|---|---|
| Average time for a customer integration | 6 to 10 weeks | 3 to 5 weeks | 1 to 3 weeks |
| Support hours per integration | 40 to 60 h | 20 to 30 h | 8 to 15 h |
| Integrations built by partners | Almost none | A few | Dozens possible |
| API infrastructure cost (gateway, logs) | 0 | EUR 300 to 600 per month | EUR 400 to 800 per month |
| Answering enterprise security questionnaires | Hard | Fair | Documented, fast |
| Credibility in RFPs | Low | Medium | Strong |
Costly mistakes
- Exposing the database schema: every internal refactor breaks customer integrations. The public contract must be decoupled.
- Forgetting versioning: a clear policy (v1, v2, deprecation announced 12 months ahead) avoids escalations.
- Webhooks without signatures or retries: a lost event means leave not synced into the customer's payroll.
- No quotas: a badly written script at 50 requests per second can degrade service for your 299 other customers.
Mini case study
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Thomas, CTO of a New York HR SaaS vendor with 300 customers, currently dedicates 1.2 engineering FTE, about EUR 78,000 loaded per year, to custom integrations and related support. He invests EUR 48,000 in the public API and EUR 12,000 in the developer portal, EUR 60,000 in total, plus EUR 600 per month of infrastructure. Integration support drops to 0.6 FTE: EUR 39,000 saved per year. Above all, 3 enterprise accounts that required an API sign for EUR 25,000 ARR each, or EUR 75,000. With EUR 114,000 of annual gains against EUR 67,200 in year one, the project pays back in about 7 months.
FAQ
How long does it take to open a public API?
Count 10 to 16 weeks for the foundation, including 2 to 3 weeks of OpenAPI contract design. The developer portal adds 3 to 5 weeks, often in parallel.
Should API access be charged?
Many B2B vendors include it in the Enterprise plan or charge a module at 10 to 20% of the subscription price. Per-plan quotas let you monetize heavy usage.
Is OAuth 2.0 mandatory or are API keys enough?
Keys are enough for server-to-server integrations of a single customer. As soon as a third-party partner accesses data from several customers, OAuth 2.0 with consent and scopes is what IT departments expect.
Do we need a penetration test before launch?
Yes, for HR data it is strongly recommended: a targeted API test costs EUR 5,000 to 10,000. Enterprise accounts often request the report in their security questionnaire.
Let's scope your project. We scope your public API (OpenAPI contract, OAuth 2.0, quotas, webhooks, sandbox, portal) for an indicative budget of EUR 30,000 to 60,000 delivered in 10 to 16 weeks. 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.
