Dungeon Defenders: First Wave cost $3 when it first came out. This week, the iOS version will cost you ninety-nine cents. But if you want to play the Unreal Engine-powered tower defense game today, you can have it for free -- developer Trendy Entertainment is now leaning on Android's new in-app billing system to pay for the whole thing. We can't give Trendy all the credit, of course, as Glu Mobile's Gun Bros and Tapulous' Tap Tap Revenge 4 are doing the same thing, but to our knowledge both of
