As an author of one semi-pro audio app, I can tell you that even the "pro" apps are selling for as little as 1/10th of the price of a similar app on a Mac. I really doubt the sales volumes compensate for that.
The big problem with the app store is that the people on the very high end of the power distribution dictate a pricing structure for everyone that only really works for those moving huge volumes. People now have a preconceived idea of what an app should cost that often has nothing to do with how much it must actually sell for in order to turn a profit.
For example, an excellent synth app like this would probably sell for at least $99 as a plugin for Mac/PC:
http://beepstreet.com/horizon
I don't know for sure but I'd be very surprised if it's moving 20x the units it would as a plugin. But if you price something like this above $10 you'll get all kinds of user outrage because that's not what an "app" should cost.
I'm listening to the link of the app and it is good but people have to be aware of it. This is the third synth app I've seen. I've also seen Animoog that was posted on The Verge.
The fault is that these apps need more advertisements. I'm willing to bet that most don't know that they can create music with a low-priced app. Consumers need to be made aware of this. There are YouTube videos destined to be made with these kind of apps.
For example, I saw an Sprint iPhone ad yesterday (maybe you've seen it?) where a guy is talking to his girlfriend while watching a game. All I gathered from the commercial was that the iPhone was on Sprint. They didn't give me the app or whether the game was live. The commercial became immediately insignificant.
I don't think marketing is the answer here. Every musician I know with an iOS device knows about this app. The problem is that the potential audience for something like this is never going to be big enough to make the Angry Birds pricing model work.
The big problem with the app store is that the people on the very high end of the power distribution dictate a pricing structure for everyone that only really works for those moving huge volumes. People now have a preconceived idea of what an app should cost that often has nothing to do with how much it must actually sell for in order to turn a profit.