I've been thinking about this as well, even though I love the whole retro thing and like supporting new developments, I still wouldn't pay more than about $20 for a new commercial Amiga game.
And obviously the game would have to be something that I actually wanted to play
There problem is I can download so many great games already for hardly any money (or for free) so it's hard to justify spending much on a new retro game in 2013.