What Does Legacy Software Modernization Cost? Realistic Numbers

TL;DR

Incremental modernization (strangler-fig style) of a typical SME system: €40,000–€150,000 over 6–18 months, while the old system keeps running. A full rewrite: 2–4x more, with a much higher failure rate. The main cost drivers are data migration, undocumented business rules, and parallel operation — not the new code.

"Our system is old, what would it cost to modernize?" has no honest one-number answer — but it has an honest framework. Here it is.

Rewrite vs. strangler fig

A rewrite builds the new system beside the old one and switches over at the end. It feels clean and fails often: the old system's undocumented behavior is the spec, and you discover it in production, after the switch.

The strangler-fig approach replaces the system one capability at a time — new modules take over routes/workflows while the legacy core shrinks. Slower on paper, dramatically safer in practice, and it delivers value from month one instead of month eighteen. This is our default recommendation, and the same logic behind technical debt you should keep: not everything old deserves replacing.

Realistic cost ranges

System size	Incremental modernization	Full rewrite
Small tool (1–3 core workflows)	€15,000–€40,000	€30,000–€80,000
Typical SME system	€40,000–€150,000	€100,000–€400,000
Large / compliance-heavy	€150,000+	€500,000+

Where the money actually goes

The new code is the predictable part. Budget for the rest:

Data migration — decades of inconsistent records, encodings, and "temporary" fields. Routinely 20–30% of the project.
Undocumented business rules — that one if branch from 2011 is somebody's critical process. Archaeology takes time.
Parallel operation — running old and new side by side, syncing data, double-checking outputs. Necessary, and never free.
The integrations nobody mentioned — the Excel export three departments secretly depend on.

When doing nothing is the right call

If the system is stable, the vendor still ships security patches, no compliance deadline looms, and it doesn't block hiring or growth — keep it. Modernization is justified by concrete pain: security exposure, an OS/runtime end-of-life, inability to integrate, or velocity that's ground to a halt. "It's old" alone is not a business case.

Want a modernization estimate for your specific system? We do a code and infrastructure assessment first, then give you a staged plan with real numbers — see our Cloud & DevOps and MVP Development services.