AppCubby, an indie developer of two popular iPhone applications – Trip Cubby and Gas Cubby – has put together a comprehensive blog post containing graphs and sales figures of their applications on the iPhone App Store. Entitled Financial Realities of the App Store, they look at the success (or not!) of advertising with MacWorld, AdMob and Google Adsense, and the effects of PR (such as a mention on Gizmodo) upon sales.
Perhaps the best conclusion, however:
[T]he only methods of marketing I’ve found to be measurably cost effective are working with the press and getting featured by Apple, both of which are essentially free, but incredibly hard to guarantee.
This continues to show that the App Store should rarely be relied upon as a marketing device in its own right – you need to do lots of legwork using good old fashioned marketing and PR to more reliably build up sales and coverage.