The numbers are all over the place, obviously because each type of app can be totally different. But how much have you been averaging per project, the last 12 months or so?
It totally depends on what client will get if you solve their issue. For example: If you are working on an app which generates $100K/month in revenue you should charge based on flat rate. Your value to the project helps them to generate revenue. Every client wants to get benefit hiring someone who can solve their issue. In this case, you will develop their bug-free app which helps them to sell on apps store and google play store. Your charges always depend on what value you provide to your clients.