Tim Worstall, in an article for Forbes:
Another number we can gain from this is that the average app, over its lifetime, earns around $9,000. That’s the mean though: the median would be much much lower. For we’ve very much got a power law going on here. A few apps are making tens of millions, one or two over $100 million, and most make nothing at all (obviously so for the free ones) or close to it even if they’re trying to charge.
The era of creating an app and instantly striking it rich is largely over. There will of course be exceptions, but it will be increasingly difficult to create an app that rises above the hundreds of thousands of apps available on the App Store. Likewise, the 'race to the bottom' for app prices makes it more difficult to make money.
It's not all gloom and doom, however. The proliferation of better tools, a plethora of open source components, and plenty of backend service providers means that it is easier and quicker than ever to create an app. The key, of course, is to come up with that next great idea and execute on it.