The Variables That Make This Question Hard to Answer
Unlike building a new application where scope drives cost, modernization cost is primarily driven by starting point. A .NET Framework application with 80% test coverage, a clean architecture, and well-documented integrations costs a fraction of a .NET Framework application with 10% test coverage, direct database access throughout the business logic, and undocumented WCF services that no one remembers writing.
Here are real cost ranges based on the modernization work we have delivered, with honest explanations of what moves the needle.
WCF to REST API Migration: $15,000 to $35,000
This range covers a small to medium WCF service with 5 to 15 operations, known callers, and reasonable documentation. The work includes: discovery and contract mapping, designing the async REST equivalents for long-running operations, building the new ASP.NET Core endpoints, running parallel operation for validation, and updating known callers.
What pushes toward $35,000: more than 15 operations, complex WS-Security or message-level encryption that needs to be replicated, unknown callers discovered during migration, long-running operations that need to be redesigned as async job patterns, or authentication migration from Windows Auth to OAuth2/JWT.
Our UN-DGACM engagement replaced WCF document indexing services with async REST endpoints backed by Azure Service Bus. The zero-downtime migration and 3x throughput improvement came from rearchitecting the synchronous processing model, not just translating the API contract.
.NET Framework to .NET 8 Migration: $30,000 to $60,000
This range covers a medium-sized application (50,000 to 150,000 lines of code) that is architecturally sound but locked to .NET Framework. The work includes: compatibility analysis (identifying APIs that changed or were removed in .NET Core), updating the project files to SDK-style, migrating from System.Web to ASP.NET Core middleware, updating any NuGet packages that have changed their API for .NET Core, and addressing async/await patterns that worked in .NET Framework but are incorrect in ASP.NET Core.
What pushes toward $60,000: heavy use of WCF (which cannot be migrated to .NET Core directly), System.Web dependencies in business logic that require architectural changes to remove, third-party libraries with no .NET Core equivalent, or complex IIS-specific configuration that needs to be replicated in ASP.NET Core middleware.
The migration is almost always worth the investment: .NET 8 offers 30 to 50% better throughput and memory efficiency versus .NET Framework on equivalent hardware, Linux deployment compatibility, containerization, and a supported security baseline.
Full Legacy Modernization: $80,000 to $200,000+
This range covers a large enterprise application with multiple services, complex integrations, low test coverage, and architectural debt that needs to be addressed alongside the platform migration. The work includes everything in the tiers above, plus: introducing a CI/CD pipeline, adding test coverage to business-critical paths (cannot safely migrate code with no tests), refactoring tight coupling to enable testability, and potentially splitting monolithic services into better-bounded components.
What defines this tier: the combination of size and starting point. A 300,000-line application with 15% test coverage and 10 undocumented integrations is a different engagement than a 300,000-line application with 70% test coverage and documented contracts. The cost is in the discovery, risk mitigation, and the additional work required to handle unknown unknowns safely.
What Drives Cost in Every Tier
- Number of integrations. Each integration must be tested against both old and new implementations during parallel operation. Three integrations with documented contracts is a different scope than 12 integrations with varying levels of documentation.
- Test coverage starting point. Lower coverage means higher risk for each change, which means more time spent writing tests before making structural changes and more time verifying behavior equivalence between old and new implementations.
- Database schema changes. If the migration requires schema changes to support new ORM patterns or different query approaches, those changes need to be applied with zero-downtime migration strategies, which add time.
- CI/CD pipeline work. Applications with no automated build or deployment pipeline require this infrastructure to be built before modernization can proceed safely. This is 1 to 3 weeks of work depending on complexity.
FriendsBit has delivered .NET modernization engagements from WCF migrations to full platform upgrades. If you are building the business case for a modernization project and need a realistic cost estimate based on your specific starting point, get in touch with the details of your application and we will give you a real number.