Considering the number of free apps that clutter the world's many app stores and the pittance paid by mobile advertising networks to app developers, it was only natural that app makers would find other ways to make money from creating mobile...
Gone are the days of writing a simple App/Game and charging $1 for it to make a million...
The App Store is now incredibly competitive and the biggest challenge is route to market and just being found.
Even when your App is found it's hard to charge for it. One, people now expect and want "free", and two, because of the very nature of an App. i.e. An App should solve a single serve need, i.e. do one thing (point solution) really well.
The challenge from a business model point of view is it's hard to charge for this and hard to defend against (someone else doing the same thing).
So instead aim to solve a *complete* customer workflow (or workflows), this is valuable, this is significant. To do this however often (not always) requires more than just an App, it requires a platform of sorts. The advantage of doing all this work is it opens the door for different business models (SaaS, etc) and is also easier to defend against.
As always, the best rule of thumb is to *solve a customer need*. If you do this, you can make money from it. Guaranteed.