The verdict in three sentences
Building an internal REST API to feed your applications costs between 15,000 and 45,000 EUR in 2026, with architecture as the main price lever. A modular monolith is enough for most mid-market firms and costs 40-50% less than microservices, which only pay off at high volume or with multiple teams. Don't underestimate OpenAPI documentation, the API gateway or authentication: these three items often make up 30% of the budget.
Monolith vs microservices: how much and for whom
A CIO is often tempted toward microservices by hype. Yet for a firm exposing 3-5 resources to a few internal apps, a well-decomposed monolith is faster, cheaper and simpler to run. 2026 ranges for Amsterdam.
| Architecture | Initial cost (EUR) | Timeline | Operations/yr | When to choose |
|---|---|---|---|---|
| Modular monolith | 15,000 - 25,000 | 8-10 wks | 3,000 - 6,000 | 1 team, moderate volume |
| Microservices (3-5 services) | 28,000 - 45,000 | 11-14 wks | 8,000 - 15,000 | High volume, multiple teams |
| Hybrid (monolith + 1-2 services) | 22,000 - 32,000 | 9-12 wks | 5,000 - 9,000 | Progressive growth |
Microservices multiply infrastructure costs (orchestration, observability, networking) and demand a DevOps team. It is a scalability investment, not a default choice.
The line items that inflate the bill
Beyond business code, a serious production API carries several mandatory building blocks. Skipping them ships a fragile API.
| Line item | Detail | Indicative cost (EUR) |
|---|---|---|
| OpenAPI/Swagger docs | Contract + dev portal | 2,000 - 4,000 |
| API gateway | Routing, rate limiting, cache | 3,000 - 6,000 |
| Authentication (OAuth2/JWT) | Tokens, scopes, rotation | 2,500 - 5,000 |
| Observability (logs, traces) | Monitoring, alerting | 2,000 - 4,000 |
| Automated tests | Unit + contract | 3,000 - 6,000 |
| Versioning & backwards compat | v1/v2, deprecations | 1,500 - 3,000 |
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Mini case study
Karim, CIO of an industrial mid-cap in Amsterdam (220 staff) wanted to expose production data to 4 internal apps without duplicating database access. We recommended a modular monolith at 23,000 EUR, 9-week timeline, over microservices at 40,000 EUR. Immediate saving: 17,000 EUR, plus 6,000 EUR/yr less in operations. The API cut future app integration time by 3x (from 3 weeks to 1 week each).
FAQ
Do I really need microservices? Rarely for a mid-market firm. They pay off at high traffic or when several teams deploy independently. Otherwise a modular monolith costs 40-50% less and is easier to maintain.
Why is OpenAPI documentation essential? Because the OpenAPI contract lets your teams and vendors integrate without endless meetings, and auto-generates clients and tests. Budget 2,000 to 4,000 EUR, easily repaid.
What is the annual operating cost? From 3,000 EUR/yr for a monolith to 15,000 EUR/yr for microservices, including hosting, monitoring and fixes. Budget this recurring cost from day one.
What is the timeline for a production API? Between 8 and 14 weeks: scoping and contract design (2 wks), development (4-8 wks), hardening, tests and docs (2-4 wks).
How do I secure an internal API? With OAuth2/JWT, per-app scopes, rate limiting at the gateway and regular audits. Even internal, an API without authentication is a major risk.
Let's scope your project. Specify the resources to expose, the number of consuming apps and expected call volume: we quote the right architecture, from 15,000 to 45,000 EUR. 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.
