I recently read a study where over 50% of all games in the App Store make less than $3,000 in their lifetime.
Very, very few people are making big bucks from a single app. And many apps that do sell that well, the budget for the app itself is huge (development costs, marketing, servers and maintenance), that they're possibly not making much profit. Where the real money seems to be (at least by looking at the Top Grossing section) are apps that are free, but where the users pay for content. I personally hate that model, but it seems to be working.
I do know a few people who've started successful businesses developing apps for other people/companies though. They'll charge a 1-time fee (or sometimes hourly rate) to do an app for someone, then move onto the next project. Or have multiple going at the same time, depending on schedules and complexity of the app. If you're good, you can make a decent living doing that.