A medium-sized app with sign up and 5-10 screen would cost about $4,000-$6,000, and will include executables for both Android and iOS. If the app is large and contains complex functionality, such as chats, audio/video playback, slick UI, the price can be $10,000 and above. However, these prices are very competitive because I develop on Flutter. My deep knowledge of the framework helps me to reduce development costs comparing to other agencies, keeping the app quality high.