The verdict in three sentences
A custom billing client area costs between 18,000 and 45,000 EUR in 2026 depending on the number of functions (payment, reminders, PDF, accounting). Expect a 3 to 6 month project and 600 to 1,800 EUR/month in maintenance. Below 200 invoices/month, a SaaS block is often enough; beyond that, or with specific business rules, custom becomes worthwhile.
Cost per function in 2026
The budget mostly depends on online payment, compliant PDF generation and integration with your accounting. Here are 2026 ranges (order of magnitude, Amsterdam agency).
| Function | Range EUR | Timeline |
|---|---|---|
| Client area + authentication | 3,000 - 6,000 | 2-3 wks |
| Invoice consultation and history | 3,000 - 6,000 | 3-4 wks |
| Compliant PDF generation (legal mentions) | 2,000 - 5,000 | 2-3 wks |
| Online payment (Stripe, GoCardless) | 3,000 - 7,000 | 3-4 wks |
| Automated reminders (email, SMS) | 2,500 - 6,000 | 2-4 wks |
| Accounting integration (Sage, Pennylane, QuickBooks) | 4,000 - 10,000 | 4-6 wks |
| Collections dashboard | 2,500 - 5,000 | 2-3 wks |
The fastest-paying function: automated reminders. They directly cut average days sales outstanding (DSO).
Custom vs SaaS block: the right trade-off
A billing SaaS block costs little upfront but comes with subscriptions and limits. Custom costs more to buy but integrates with your stack.
| Criterion | Custom module | SaaS block |
|---|---|---|
| Upfront cost | 18,000 - 45,000 EUR | 0 - 2,000 EUR setup |
| Monthly cost | 600 - 1,800 EUR maintenance | 40 - 300 EUR/month + % transaction |
| Customisation | Full (rules, brand, workflow) | Limited to options |
| Accounting integration | Native, on your tool | Standard connectors |
| Data ownership | Complete | Vendor lock-in |
| Worthwhile from | ~200 invoices/month | Small volumes |
The threshold sits around 200 invoices/month or when your billing rules (subscriptions, pro-rata, discounts) fall outside SaaS standards.
Mini case study
Julian, head of a maintenance SME in Amsterdam, issues 420 invoices/month and suffers a 52-day DSO. His manual reminders take 0.5 FTE (18,000 EUR/year loaded). A 31,000 EUR client area (payment + reminders + accounting) automates collections. By cutting DSO to 34 days, he durably frees about 68,000 EUR of cash and saves the 0.5 FTE. On reminder costs and avoided interest alone, payback is around 20 months.
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FAQ
At what volume is custom worthwhile?
Generally above 200 invoices/month, or sooner if your billing rules (subscriptions, pro-rata, complex discounts) do not fit a standard SaaS.
How much does online payment cost to integrate?
Between 3,000 and 7,000 EUR for Stripe or GoCardless, excluding transaction fees (about 1.4% + 0.25 EUR per European card in 2026).
Can we connect our accounting software?
Yes. Integration with Sage, Pennylane or QuickBooks costs 4,000 to 10,000 EUR and removes double entry between billing and accounting.
Are automated reminders really useful?
Yes: they are what lowers DSO. A 3-reminder scenario (D+3, D+10, D+20) typically cuts payment time by 10 to 20%.
How long to go live?
A full client area ships in 3 to 6 months depending on functions. An invoices + payment base ships in 3 months.
Let's scope your project. Give us your invoice volume, your accounting tool and your reminder rules, and we will frame a client area from 18,000 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.
