What RPG and COBOL modernization projects actually cost, what drives the price, and how modernization compares economically to a full ERP replacement.

Modernization cost estimates vary enormously by scope -- and scope is almost entirely a choice, not a fixed technical requirement. Two published benchmark ranges illustrate the spread.

Published modernization cost benchmarks

Scope	Cost Range	Timeline
Large enterprise IBM i RPG/COBOL modernization	$5,000,000 - $20,000,000+	12-24 months
General enterprise system modernization (cross-industry)	$2,200,000 - $4,000,000	12-24 months, 12-24mo breakeven

Ranges reflect real reported project figures, not a single controlled study. Always validate against a scoped proposal.

What actually drives the cost

Codebase scoping

Targeting the roughly 70% of the codebase that actually delivers business value, instead of rewriting everything, is the single biggest lever on cost, timeline, and risk.

Big-bang vs incremental

A full-rewrite approach concentrates cost and risk upfront with little room to adjust mid-project. Incremental modernization spreads cost but extends the overall timeline.

Database/OS coupling

RPG's tight integration with the IBM i database and OS makes decoupling business logic from data the hardest technical hurdle -- and the biggest source of underestimated effort.

Testing budget

Allocate roughly 40% of budget to user acceptance testing rather than the commonly budgeted ~20% -- underinvesting here shows up as post-launch defect cost instead.

Modernization vs. replacement: two different bets

RPG/COBOL modernization keeps your existing IBM i platform and business logic, re-platforming or re-facing it for maintainability and integration. A full ERP replacement (SAP, Oracle, or a cloud ERP) replaces the platform and the logic together. Modernization is generally the lower-risk, lower-disruption path when your custom logic represents real competitive advantage; replacement can make sense when the logic itself is largely commodity process that a packaged ERP already handles well. See our Competitive Software TCO page for the replacement-side cost comparison.

Modernization cost estimates without a code-level scoping exercise are little more than guesses. Before budgeting against either end of the published range, get a data-driven analysis of your actual codebase size and complexity.

IBM i Application Modernization Cost