Current app store sizes:
|Name of "app" store||Number of apps|
|Apple App Store1||1,200,000|
|Windows Phone Store1||300,000|
|Amazon App Store1||240,000|
|Chrome Web Store2||~34,000|
|Chrome Apps on CWS3||2,704|
Sources: ,  (includes extensions),  (by my count on May 6, 2015), 
There were 91 paid Chrome Apps in the Chrome Web Store. Ignoring the 3 paid apps that had monthly charges, the average paid app had an estimated net revenue of $2,419. (Net revenue can only be estimated as one doesn't know if the app's price changed over time, if the number of users includes those that uninstalled, or if free copies to beta testers are included in the number of users.) There were only 4 apps that made more than $10,000. For details, see my spreadsheet. I have no information about revenue from in-app purchases.
There were 428 paid apps in Firefox Marketplace. No user/purchase count is available, so I did not attempt an individual breakdown. Until recently, the marketplace was really only for Firefox OS phones, so naturally the bulk of the apps there are aimed at phones. Interestingly, Microsoft has several apps in the store. In particular, Bing Maps seems popular on the platform.
So the long and the short here is that there is currently not much of a market for "Desktop Web Apps" outside of the Windows Store. That said, Google has done an amazingly poor job of creating one. I'll have more to say about that in a later post.