The verdict in three sentences
A custom REST API to interconnect your website, tools and ERP costs in 2026 between 15,000 and 40,000 EUR (roughly GBP 13,000-34,000), delivered in 6 to 12 weeks with OAuth2 authentication, OpenAPI documentation and tests. An iPaaS (integration Platform as a Service) skips the build but costs 500 to 2,000 EUR/month and locks you into its connectors. The choice turns on three criteria: flow volume, business specificity and your 3-year ownership horizon.
Custom API vs iPaaS: the real 3-year math
An iPaaS (Make, Workato, or a cloud-native block) is fast to set up when your flows are standard. A custom API becomes worth it as soon as your business rules are specific or transaction volume makes usage-based pricing explode.
| Criterion | Custom API | iPaaS |
|---|---|---|
| Upfront cost (EUR) | 15,000 - 40,000 | 2,000 - 6,000 (setup) |
| Recurring cost | 300 - 800/mo (maintenance) | 500 - 2,000/mo |
| 3-year total (order of magnitude) | 26,000 - 69,000 | 20,000 - 78,000 |
| Business customization | Full | Limited to connectors |
| Vendor lock-in | Low | High |
| Scaling | Controlled | Billed by volume |
At moderate volume and standard flows, iPaaS wins year one. Once you pass tens of thousands of calls/month or need specific mapping rules (per-customer pricing, multi-warehouse stock, multi-country VAT), the custom API regains the economic and strategic edge.
What makes up a custom API budget
A robust API is not just a list of endpoints: it is security, documentation and tests. Here is a 2026 breakdown for an ERP integration project.
| Item | Person-days | Indicative cost (EUR) |
|---|---|---|
| Flow scoping + data model | 4-7 | 2,500 - 4,500 |
| REST endpoints + business logic | 12-20 | 7,000 - 13,000 |
| OAuth2 auth + scope management | 4-6 | 2,500 - 4,000 |
| OpenAPI documentation + portal | 2-4 | 1,500 - 3,000 |
| Automated tests + sandbox | 4-6 | 2,500 - 4,000 |
| Monitoring + error handling | 3-5 | 2,000 - 3,500 |
Then budget maintenance of 300 to 800 EUR/month: security updates, quota monitoring, adapting to ERP changes. An API without monitoring or fine-grained error handling becomes unmanageable at the first ERP outage.
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Mini case study
Sarah, CIO of a B2B distribution firm in London, must sync her e-commerce site, CRM and ERP (stock, per-customer pricing, orders). An iPaaS is proposed at 1,400 EUR/month. A custom API is quoted at 32,000 EUR plus 600 EUR/month maintenance. Over 3 years: iPaaS = 50,400 EUR; custom API = 32,000 + 21,600 = 53,600 EUR, but with per-customer pricing rules impossible to implement in the iPaaS. Since those rules generate 8 % additional margin on 4 M EUR of flow, the API pays back in under a year. Decision: custom API, with a staging sandbox before cutover.
FAQ
How long to deliver an ERP integration API? Between 6 and 12 weeks depending on the number of endpoints and mapping complexity. A simple connector (catalog + stock) ships in 4 to 6 weeks.
Is OAuth2 mandatory? For a B2B API exposing sensitive data (pricing, orders), OAuth2 with scopes is the 2026 standard. A simple API key suffices for non-exposed internal flows but limits traceability.
What happens if the ERP changes version? A well-designed API isolates the ERP behind an abstraction layer: a version upgrade is handled in 2 to 5 maintenance days rather than a full rebuild.
iPaaS or custom to start fast? iPaaS ships in 2 to 4 weeks for standard flows. If your rules are specific or volumes high, custom avoids costly lock-in in the medium term.
Do you need OpenAPI documentation? Yes: it lets your teams and partners integrate without support, cuts tickets and secures changes. Budget 1,500 to 3,000 EUR, easily recouped.
Let's scope your project. Tell us your target ERP, your priority flows and expected call volume, and we will price the API and its maintenance plan. 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.
