How much does it cost to build a MERN SaaS application?

A MERN SaaS application costs $50,000 to $120,000 for a production single-tenant or light multi-tenant SaaS (4 to 7 months), or $80,000 to $250,000 for a proper multi-tenant MERN SaaS with subscription billing, analytics dashboards, and 3 to 5 integrations (6 to 12 months). Add $20,000 to $50,000 for enterprise SSO, compliance logging, and audit trails. US or UK agencies charge 2 to 3 times these ranges for the same MERN SaaS scope.