Fixed bid works just fine with an agile approach. You just start with a very small core project, then iterate upwards with follow on projects.
This is especially important for the Apple App Store because there is no other way to know if your app will be approved.
Fixed bid is sometimes more expensive due to the "risk eating" the developers are taking off your hands, but agility isn't usually the issue with developers who are otherwise good.
We counsel clients to start with a small fixed bid project then use hourly or follow on projects to change the program to a particular place they want to go.