Developer Owen Goss (the mind behind Landformer and Baby's Musical Hands) has spent the last few weeks or so putting together a really comprehensive survey on revenue for iOS games and apps, all based on answers from 252 actual iOS developers. He's shared the results on his blog, and they're pretty fascinating. Unfortunately, as he states early on, this is a pretty informal survey, with a self-selected sample group that's on its honor to answer truthfully (and some of the questions, Goss admits, may not have been completely clear).
But there's still some interesting info in here. The majority of the developers polled are working independently, and a surprising amount of them, exactly 50%, are working on their own as a one-person shop. And while some developers have made a lot of money on the store (the average lifetime earnings is actually around $165k), most developers don't make more than a few thousand dollars. 50% of developers polled in Goss' survey have made less than $3,000 in all of their app sales. The median game, too, made less than $1000, though of course there were some games that made much less and much more.
Goss also found a few other interesting stats, like the fact that most developers tend to make more revenue as they release more games (which is logical -- of course devs get better at making, marketing, and selling games as they get more experienced at it). If you're a developer, an aspiring dev, or just interested in how the iOS dev grind goes, you'll want to read through the results of the survey.