The verdict in three sentences
Opening a public API to 50 integrators costs a London SaaS vendor roughly 40,000 to 110,000 GBP in year one, including design, portal and gateway. The most underestimated line is OpenAPI contract design and versioning: a poorly designed API costs twice as much to evolve without breaking integrations. The developer portal does not need to be lavish, it needs to get an integrator to a first successful call in under 15 minutes.
Budget lines for a public API in 2026
The CTO of a SaaS vendor gets partner requests every month to sync data. As long as access runs through CSV exports or ad hoc connections, each integration ties up the team. A documented public API turns that cost into a commercial lever.
| Line item | 2026 range (excl. VAT) | What drives the price |
|---|---|---|
| OpenAPI design, resource model, versioning | 13,000 to 35,000 GBP | Number of exposed resources (10 or 60), refactoring existing code |
| OAuth2 authentication, API keys, scopes | 4,500 to 10,000 GBP | Client credentials only, or authorization code for third-party apps |
| Developer portal (docs, console, sign-up) | 9,000 to 26,000 GBP | Customized open source tool or custom portal |
| API gateway (quotas, logs, analytics) | 0 to 1,700 GBP/month | Self-hosted or managed, call volume |
| Outbound webhooks and retries | 3,500 to 9,000 GBP | Message signing, retry queue, delivery log |
| Client SDKs (1 to 3 languages) | 2,500 to 7,000 GBP per language | Generated or hand-written SDKs |
| Sandbox with test data | 2,500 to 8,000 GBP | Data isolation, automatic reset |
2026 order of magnitude for an agency or external team. London day rates for senior contractors run around 550 to 800 GBP, and a senior in-house backend developer costs roughly 85,000 to 110,000 GBP a year in salary.
API gateway: self-hosted or managed
The gateway handles quotas, rate limiting, keys and logs. For 50 integrators and a few million calls per month, both approaches are defensible.
| Option | Monthly cost | Advantages | Limits |
|---|---|---|---|
| Self-hosted open source gateway | 90 to 350 GBP (infrastructure) | Low cost, full control | You run upgrades and operations |
| Public cloud gateway | 170 to 1,000 GBP depending on volume | Built into existing hosting | Cloud dependency, per-call billing |
| Full managed API platform | 700 to 1,700 GBP and up | Portal, analytics and monetization included | Cost climbs with call volume |
| Rate limiting in application code | 0 GBP recurring | Enough to start | Little visibility, no portal |
| Quotas per plan (free, partner, premium) | Included in the options above | Basis for future monetization | Requires well-defined plans |
For a first launch to 50 integrators, an open source gateway or your cloud's native one is enough. A managed platform becomes useful beyond 200 to 300 active integrators or if the API is billed.
What makes a public API succeed
Three technical choices prevent most problems: explicit versioning from v1 (in the URL or a header), a written deprecation policy with 6 to 12 months' notice, and structured, documented error messages. Under UK GDPR, the vendor remains responsible for personal data exposed through the API: OAuth2 scopes must limit access to what is needed and logs must record who read what. Finally, track "time to first successful call": it is the metric that best predicts adoption.
Mini case study
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Oliver, CTO of a 40-person HR SaaS vendor in London, handles 50 partner integrations manually. Each takes an average of 6 development days per year, so 300 days at 500 GBP loaded cost, or 150,000 GBP a year. The public API project costs 70,000 GBP plus 500 GBP per month for the gateway. If 70 % of partners become self-serve, savings reach 105,000 GBP a year, and the project pays back in about 9 months.
FAQ
REST or GraphQL for a public API?
REST with OpenAPI remains the simplest choice for 50 integrators with varied profiles. GraphQL makes sense if partners need very flexible queries, and adds 15 to 25 % implementation complexity.
How long to launch v1?
Allow 3 to 5 months for an API of 15 to 25 resources with portal and sandbox. A staged opening to 5 pilot partners for 6 weeks sharply reduces post-launch fixes.
Should we charge for API access?
At launch, most vendors include it in higher plans to reduce churn. Usage-based billing usually comes later, often beyond 100,000 calls per month per customer.
How do we handle a breaking change?
Publish a v2 in parallel, announce v1 end of life with 6 to 12 months' notice, and use logs to see which customers still call the old version.
What security level is needed?
OAuth2, TLS, rate limiting and key rotation are the minimum. A focused API penetration test costs about 5,000 to 13,000 GBP in London.
Let's scope your project. Share your API's functional scope, target number of integrators and stack for a costed plan covering design, portal and gateway. 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.
