I was told by one there app pays for the mortgage of their house.
Red Storm has the sense of it: it's either a little or a lot, depending on the app.
Most of them don't make much money at all, whether they're not very good or just aren't getting much visibility. Having a well-designed photo sharing app doesn't matter if you're 539th in the Photo & Video category, and a niche or me-too app rarely does well.
However, there is a subset of app developers that are genuinely successful. It's rare that you find a lone person so successful that they're wealthy from their apps, but numerous small outfits make a good living from their work, like Taptaptap (Camera+) and Tapbots (Tweetbot). And of course, there are a few companies that hit it big, such as King (Candy Crush Saga) and Machine Zone (Game of War).