Have a look a this article (also linked by John Gruber ), that is based on aggregated data:
Overall we estimate that $6,000,000 has been paid out to developersOf course it depends on the app, but the data and the consumer behavior patterns of android users show it's unlikely you become rich with a single android app, no matter how successful it gets.
for games, and $15,000,000 has been paid out on apps. That is a total
of $21,000,000, almost 50x lower than the amount paid out to devs on
From what I have read about android app development, the money is in running advertising in your app. I would study search patterns and the money that follows through a keyword tool like Google's, https://adwords.google.com/select/KeywordToolExternal.
This will give you a feel for where the money may be at. If you can infer who a particular app's audience is and the developer is using advertising as its source of revenue, you may be able to get a feel for how much is being made.
Check out this guy's blog, http://www.kreci.net/. He chronicles his experience as an android developer; it's pretty good stuff.
It's like any industry (esp actors or models) ... 1% make all the cash.. 99% have "dreams". So chances are any app out there is not making enough money.
My research on the app store is that a "normal" app makes (naturally) around $500 a year.
Angry Birds, on the other hand, has made $70 million.
Once you have one successful app - you can make any app successful provided you have done the "right" things and are able to market to existing users.
DragonVale on the app store is being promoted by the guys who did Paper Toss. When you have 20 million users - getting another million to buy an app is a piece of cake.
My research has shown that it's pretty easy to get 1 million users for any app. of course... if you have money to spend.
How to tell if a developer is doing well? have a look at how many apps they have on the store / platform.. Look at how many reviews, have a look at the leader boards to see how many people have logged high scores. This will give you an idea.