The verdict in three sentences
A custom ERP API connector costs between 12,000 and 25,000 EUR and ships in 5 to 9 weeks, excluding the e-invoicing layer. E-invoicing compliance (mandatory in stages across the EU in 2026) adds 6,000 to 11,000 EUR depending on your formats and certified platform. The real trade-off is not the upfront price but the annual maintenance cost: an iPaaS bills 200 to 800 EUR/month, while a custom build carries a heavier upfront cost but a lighter recurring one.
What a custom ERP connector covers
An ERP integration is not just "pushing invoices". It syncs customers, items, journal entries, payments and statuses in both directions, handles errors and replays failed flows. In Amsterdam, an SME of 20 to 80 staff typically wires its business app to the ERP via REST APIs.
| Item | Scope | 2026 Cost (EUR) | Timeline |
|---|---|---|---|
| Base API connector | Customers + items + invoices | 12,000 - 16,000 | 4 - 6 wks |
| Two-way sync | Payments + statuses + credit notes | 16,000 - 21,000 | 6 - 8 wks |
| Complex multi-entity flow | Analytics + multi-rate VAT | 21,000 - 25,000 | 7 - 9 wks |
| E-invoicing layer | Formats + platform submission | 6,000 - 11,000 | 3 - 5 wks |
| Historical migration | Mapping + data acceptance | 2,500 - 5,000 | 1 - 2 wks |
An experienced integrator's daily rate sits between 480 and 620 EUR in 2026 across the Benelux.
Point-to-point vs iPaaS: the real TCO match
The question that decides your 3-year budget: custom point-to-point dev or an integration platform (managed iPaaS)? An iPaaS speeds up go-live but installs a lifetime subscription.
| Criterion | Custom point-to-point | iPaaS / middleware |
|---|---|---|
| Upfront cost | 12,000 - 25,000 EUR | 5,000 - 12,000 EUR |
| Monthly subscription | 0 EUR (hosting only) | 200 - 800 EUR/mo |
| Annual maintenance | 1,800 - 4,000 EUR | partly included |
| Go-live time | 5 - 9 wks | 3 - 6 wks |
| Vendor dependency | Low | High (lock-in) |
| 3-year cost (ballpark) | 17,000 - 34,000 EUR | 17,000 - 41,000 EUR |
Below 3 or 4 stable flows, custom wins over 3 years. Above 8 to 10 fast-changing flows, the iPaaS pays back its subscription through saved integration time.
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Mini case study
Claire, CFO of a 45-person industrial SME in Amsterdam, issues 320 invoices/month re-keyed by hand into the ERP. Hidden cost: about 22 hrs/month of double entry, roughly 8,400 EUR/year in admin time. She invests 19,000 EUR in a two-way connector plus 8,000 EUR for the e-invoicing layer. Re-keying drops to 3 hrs/month. Annual time saving ~7,300 EUR, plus secured 2026 compliance: the connector pays back in under 30 months, before counting fewer VAT errors.
FAQ
Is e-invoicing really mandatory in 2026? Yes, in stages: reception first for all businesses and issuance for large firms and mid-caps, then SMEs and micro-businesses later. Building the connector early avoids a rushed project under calendar pressure.
Do I need a certified platform? In most cases yes: the certified platform transmits your invoices in the compliant format. The connector must therefore talk to both the ERP and your platform, which represents 6,000 to 11,000 EUR of dedicated development.
How much is annual maintenance for a custom connector? Budget 1,800 to 4,000 EUR/year to track ERP API changes, regulatory updates and new cases. That is clearly less than an iPaaS subscription at 400 EUR/month.
How long before production? Between 5 and 9 weeks for a standard connector, plus 3 to 5 weeks for the e-invoicing layer. A historical migration adds 1 to 2 weeks of acceptance testing.
Can we start small then extend? Yes: we ship customers + invoices first (12,000 - 16,000 EUR), then add payments, credit notes and analytics in batches. This spreads spending and validates each flow before the next.
Let's scope your project. Tell us your ERP version, invoice volumes and compliance deadline, and we'll frame the connector and its e-invoicing layer. 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.
