There are two on Google Play — DOSBox Turbo and "DOSBot". Both charge a fee — DOSBox Turbo is $3.99; DOSBot is $0.99. The developer of DOSBot says on his Google Play entry that he will not release the source code of his application because it's not GPL, even though it's derived from source released under GPL v2 — this is definitely a violation of the license. The developer of DOSBox Turbo is refusing to release the source for his application unless you pay the $3.99 to "buy" a license of it.
The same developer explicitly states that the "small" fee (although one might argue that $3.99 is pretty expensive for an OSS Android app) is to cover the cost of development. Unless I'm misreading the text of GPL v2, a fee can only be charged to cover the cost of the distribution of a program or derived work, not the cost of development. And, of course, it doesn't cost the developer anything for someone to log in to Google Play and download their app. In fact, from what I can tell, there's a one-time $25 fee to register for Google Checkout, after which releasing apps is free.
Where do you draw the line on this? What do you do in this kind of situation?