As others have pointed out, every application and developer is a different story, just as they are for desktop software products. You can develop an approximate feel for a marketplace, but how your particular application will sell depends entirely on what it does, its price, the overall user experience, whether or not you are promoted by the platform holder, how you advertise, and how you support the application, among many other factors. Unless you find a case study that is exactly like your own intended application, exact sales figures might not do you more good than the approximate numbers already out there.
I've been very open with my own experiences in selling an iOS application, which I describe in detail in the "Selling iOS Applications" session from my advanced iOS development course. The video for that class is available for free on iTunes U. Additionally, I had three guest speakers talk in the previous semester, each with successful applications that used different revenue models: Steve Glinberg with for-pay educational applications like KidCalc, Justin Beck with their in-app-purchase-supported MMORPG Parallel Kingdom, and Neil Mix with the ad-supported Pandora application. Steve and Justin go into great detail about how their applications have performed in their areas, including financial details.
I lay out my personal take on the App Store as a market in the course notes.
For about the last two years, I have been selling a $10 visual calculator called Pi Cubed on the App Store. To date, its net revenue (after Apple's cut) is $50,700. It currently brings in a sustainable ~$1,700 in net revenue a month, which is off its high of about $3,500 a month simply due to the fact that I have not sustained regular updates of it lately (my day job has cut into my time for this).
This application has never appeared on any top list, but I've sustained sales through the use of regular advertising. At a $10 price point, I can use banner ads to drive regular sales at a net profit, and therefore do not need to rely on spikes in sales to get on the top charts or on Apple's grace in promoting the product.
I've experimented with making money through advertising using iAds in a free Lite version of this application. However, I didn't want to crap up the main interface, so the ads are somewhat hidden on a secondary screen within the application. Since October, over 60,000 people have upgraded to or downloaded the version of this application with iAds. In that period, I have seen 54,000 ad requests, 7,200 impressions, and $116 in revenue. This is significantly less than the $12,000 my for-pay version made over that same period. Again, I didn't have the most in-your-face ad placement in this application and it isn't a game that you spend hours within, so another class of application may do far better.
I have no experience with any other mobile platform, so I can't comment on how Android, WebOS, or BlackBerry compares with this for my case. Anecdotal evidence seems to indicate that it is significantly more difficult to get users to pay for applications on Android, but that advertising works well there. WebOS hasn't taken off yet, and the one company I know with a product on the store there has been very disappointed with the platform. I don't know any current BlackBerry developers, because the ones I did know moved to Android or iOS. The Playbook may change this.
