Flutter is an open-source framework, but building a Flutter application requires an experienced development team.
The cost depends on project scope, delivery time, app complexity, design requirements, integrations, platform-specific features and level of customisation. A simple mobile MVP will have a different budget from a larger application connected with APIs, payments, user accounts, analytics, notifications and internal business systems.
Fixed price works when the scope is clearly defined. Time and materials gives more flexibility when requirements may change during development.
A reliable estimate starts with understanding your users, product goals, app features, integrations and technical requirements.