The verdict in three sentences
For a B2B software vendor, opening a serious public API costs 20,000 to 50,000 EUR in design and security, plus 10,000 to 25,000 EUR for a developer portal with OpenAPI documentation and key management. Add a managed API gateway at 300 to 1,500 EUR per month depending on traffic. The investment pays back mostly through retention: a customer who has wired their ERP or CRM into your API churns far less.
Why a public API changes a software vendor's economics
A B2B software vendor in Amsterdam sooner or later gets the same request: "can you connect to our HR system, our CRM, our BI tool?". Without a public API, each request becomes a one-off build, billed or given away, that adds to maintenance. With a documented API, customers, integrators and partners build the connectors themselves.
| Indicator (2026 order of magnitude, B2B SaaS) | Without public API | With public API and portal |
|---|---|---|
| Annual churn of integrated customers | 10 to 14% | 5 to 8% |
| Time to deliver a customer integration | 4 to 8 weeks (in-house team) | 1 to 3 weeks (customer or integrator) |
| Custom integration requests per year | 15 to 30 | 3 to 8 |
| Average cost of a custom connector | 8,000 to 20,000 EUR | close to zero for the vendor |
| Share of enterprise deals requiring an API | 60 to 80% | requirement met |
| Active technology partners | 0 to 3 | 10 to 30 within 2 years |
The first effect is commercial: in enterprise tenders, the lack of a documented API often eliminates a vendor at the shortlist stage. The second is financial: every churn point saved on a 3 M EUR ARR base preserves 30,000 EUR of recurring revenue a year.
What to build and what it costs
A public API is not just exposing internal routes. It needs a dedicated, versioned, secured layer with rate limits and documentation an external developer can understand in under an hour.
| Component | Scope | 2026 budget (order of magnitude) |
|---|---|---|
| REST API design | resource model, versioning, conventions, OpenAPI contract | 8,000 to 18,000 EUR |
| Security | OAuth 2.0, scopes, per-client keys, logging, targeted penetration tests | 7,000 to 17,000 EUR |
| Webhooks and events | subscriptions, signatures, replay | 5,000 to 15,000 EUR |
| Developer portal | interactive docs, sandbox, key creation, changelog | 10,000 to 25,000 EUR |
| Managed API gateway | quotas, rate limits, analytics | 300 to 1,500 EUR per month |
| Client SDKs (2 languages) | generated and tested libraries | 4,000 to 10,000 EUR |
| Support and maintenance | changes, deprecations, monitoring | 1,000 to 2,500 EUR per month |
A credible core (API, security, portal) sits between 25,000 and 60,000 EUR. Webhooks and SDKs can come later, once first usage is observed.
Build, buy or combine
| Option | Upfront cost | Annual cost | For whom |
|---|---|---|---|
| Static docs generated from OpenAPI | 3,000 to 8,000 EUR | low | private API for a few partners |
| Off-the-shelf SaaS developer portal | 2,000 to 6,000 EUR | 6,000 to 30,000 EUR | small team, quick need |
| Custom developer portal | 10,000 to 25,000 EUR | 6,000 to 15,000 EUR | strong brand, onboarding built into the product |
| Self-hosted open source gateway | 5,000 to 12,000 EUR setup | 4,000 to 10,000 EUR operations | DevOps team available |
| Managed cloud gateway | low | 3,600 to 18,000 EUR | most vendors |
The most common combination in 2026 among vendors with 20 to 80 staff: managed gateway, custom portal built into the existing customer area, OpenAPI contract as the single source of truth.
Mini case study
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Illustrative scenario: Joost, CTO of an HR SaaS vendor in Amsterdam, runs 3.2 M EUR ARR with 420 customers. His annual churn is 11%, and his team ships 20 custom connectors a year at an average internal cost of 12,000 EUR, i.e. 240,000 EUR.
He plans 45,000 EUR for the API and portal, plus 18,000 EUR a year for the gateway and maintenance. If he brings custom connectors down to 6 a year, he saves 168,000 EUR. If churn drops by 2 points among integrated customers (about 40% of the base), he preserves roughly 25,600 EUR of ARR a year. The project pays back in under 4 months on connector savings alone.
FAQ
How long does it take to launch a public API and its portal?
Plan 10 to 16 weeks for a core with OAuth security and a developer portal. Designing the OpenAPI contract alone takes 3 to 4 weeks.
Should we charge for API access?
Many vendors include it in top-tier plans, which pushes 10 to 20% of customers to upgrade. Usage-based billing makes sense beyond a few million calls per month.
How do we secure an API open to third parties?
OAuth 2.0 with scopes, per-client rate limits, logging and a targeted 3 to 5 day penetration test before launch. That test usually costs 4,000 to 8,000 EUR.
How do we version without breaking customer integrations?
Version in the URL or header and announce every deprecation at least 6 to 12 months ahead. The portal changelog plus an email to key holders is enough in most cases.
Does GDPR change anything for a public API?
Yes, the API must respect the data processing agreement: minimal scopes, access traceability and data deletion. Budget 2 to 4 days of legal and technical analysis.
Let's scope your project. Share your stack, your 5 most requested integrations and your expected call volume, and we will price the API, portal and gateway with a launch timeline. 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.
