Flutter is an open source mobile app development platform introduced by Google in 2015 that allows developers to create visually attractive and fast mobile apps for both Android and iOS using one codebase. The cost of developing a Flutter app ranges from $15,000 to $40,000 depending on factors like the developer's hourly rate and the complexity of the app. Choosing Flutter over other platforms can save developers time since only one codebase needs to be written and tested. Maintaining a Flutter app costs around 20% of the initial development cost annually to address bugs, design changes, and app updates.