Audiobus: Use your music apps together.
What is Audiobus? — Audiobus is an award-winning music app for iPhone and iPad which lets you use your other music apps together. Chain effects on your favourite synth, run the output of apps or Audio Units into an app like GarageBand or Loopy, or select a different audio interface output for each app. Route MIDI between apps — drive a synth from a MIDI sequencer, or add an arpeggiator to your MIDI keyboard — or sync with your external MIDI gear. And control your entire setup from a MIDI controller.Download on the App Store
Audiobus is the app that makes the rest of your setup better.
Chris Randall Interview
A lot of this is quite technical, but there's an interesting discussion on app pricing on the end.
TLDR - basically it's very hard to make money on any platform, and Chris says he only really has a viable business because he can generate plugins for OSX/Windows/IOS. Audio plugins just aren't a big market, even for someone as established as Chris.
Also - while he grumbles about IOS pricing, he states that he makes about the same amount of money on IOS apps due to how big the market is, as he does on the other platforms (despite charging 10x as much). He seemed reasonably okay with IOS as a platform.