The verdict in three sentences
Building a partner REST API documented with OpenAPI for a B2B platform costs, as a 2026 ballpark, 13,000 to 38,000 USD excl. GST, developer portal and automated tests included, for a timeline of 8 to 14 weeks. On top of that come 55 to 270 USD per month of monitoring and a few dozen dollars of API gateway depending on request volume. The return comes from ending ad hoc integrations partner by partner: a well-documented integrator connects in a few days instead of several weeks.
The detailed budget of a partner API
An API open to third parties is very different from an internal API: it must be stable, versioned, secured per client and documented well enough that an external developer can use it without calling your team. The OpenAPI 3.1 contract is written first, then used to generate documentation, tests and sometimes SDKs.
| Component | Content | Indicative effort | 2026 ballpark (USD excl. GST) |
|---|---|---|---|
| OpenAPI contract design | Resource modelling, errors, pagination, conventions | 5 to 10 days | 1,600 to 3,800 |
| REST endpoint development | 10 to 25 resources, filters, outgoing webhooks | 20 to 50 days | 6,500 to 18,400 |
| Authentication and API keys | OAuth 2.0 client credentials, key rotation, scopes | 4 to 8 days | 1,300 to 3,200 |
| Quotas, rate limiting, versioning | Per-partner caps, v1 and v2 handling | 3 to 6 days | 1,100 to 2,700 |
| Developer portal | Interactive docs, sandbox, partner sign-up | 6 to 12 days | 1,600 to 5,400 |
| Automated testing | Contract tests, load tests, continuous integration | 5 to 10 days | 900 to 4,300 |
A first version limited to 10 read-only resources, with Redoc or Swagger UI documentation, lands around 13,000 to 17,000 USD excl. GST. A full read and write API with webhooks, sign-up portal and SDKs approaches 32,000 to 38,000 USD excl. GST. These figures reflect a nearshore delivery team; a fully Singapore-based team typically quotes 1.5 to 2 times more.
Gateway, portal and operations: the choices that drive recurring cost
| Item | Budget option | Mid-range option | Advanced option |
|---|---|---|---|
| Documentation | Redoc or Swagger UI, free | Custom static portal, 0 to 30 USD per month hosting | Specialised SaaS portal, 110 to 430 USD per month |
| API gateway | Rate limiting built into the code | Managed cloud gateway, 30 to 160 USD per month | API management platform, 320 to 1,080 USD per month |
| Monitoring | Uptime alerts, 55 USD per month | Per-partner metrics and alerts, 130 USD per month | Distributed tracing and dashboards, 270 USD per month |
| API hosting | 85 to 160 USD per month | 160 to 320 USD per month | Multi-zone, 430 USD per month and up |
| Partner support | Dedicated email inbox | FAQ and changelog area | Dedicated channel with response commitment |
For 10 to 30 partners, the mid-range option is more than enough: allow 320 to 650 USD per month in total. Full API management is only justified beyond several hundred integrators or when monetising the API.
Rules that avoid breaking your partners
- Explicit versioning: any breaking change goes into a new version, the old one maintained for 6 to 12 months with a changelog announcement.
- Standardised errors: a single format (for example RFC 9457) with code, message and request ID, so partners can diagnose on their own.
- Signed webhooks: each outgoing notification carries an HMAC signature and is replayed on failure.
- Compliance: if the API exposes personal data, Singapore's PDPA applies, and each partner signs terms of use that govern the data.
- Penetration test before opening: a 3 to 6 day API test limits the risk of a leak through a misconfigured partner.
Mini case study
Wei Ling, CTO of a B2B logistics services platform in Singapore (illustrative persona), maintains 12 ad hoc partner integrations (CSV exports, custom scripts). Each consumes about 1 developer day a month in fixes, at 380 USD a day.
- Current cost: 12 x 380 USD, so 4,560 USD a month and 54,720 USD a year.
- Documented partner API: 26,000 USD excl. GST, plus 216 USD per month for monitoring and gateway.
- Assumption: 70% of maintenance effort disappears once partners are migrated, so 38,300 USD saved a year, minus 2,590 USD of running costs.
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Net savings reach about 35,700 USD a year, paying back the project in under 9 months. Bonus: onboarding a new partner drops from 6 weeks to about 1 week.
FAQ
REST or GraphQL for a partner API?
REST with OpenAPI remains the standard most B2B integrators expect and is easier to document. GraphQL adds 15 to 25% effort on security and quotas, best kept for cases where partners have very varied data needs.
Do we need a developer portal from version one?
Interactive documentation generated from the OpenAPI contract is enough for the first 5 to 10 partners. A portal with self-service sign-up becomes useful beyond that, for a budget of 1,600 to 5,400 USD.
How do we secure each partner's access?
Each partner gets its own OAuth 2.0 credentials, limited to its scopes and a quota, for example 60 requests per minute. A compromised key is revoked without affecting the others.
How long does it take to deliver the API?
Allow 8 to 14 weeks: 2 weeks of contract design validated with 2 or 3 pilot partners, 5 to 10 weeks of development and 1 to 2 weeks of beta.
Can we monetise the API?
Yes, with subscription tiers by request volume, but that requires an API management platform (320 to 1,080 USD per month) for usage-based billing.
Let's scope your project. Describe the data to expose, the number of target partners and your security constraints: we will price the OpenAPI contract, development and operations with an indicative budget and schedule. 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.
