I dont have one, but I occasionally get on the android central forums and I think I remember a thread dedicated to invite sharing over there. You might give it a try. I think they just ask once you get yours that you share some of the invites you get with the group.
It totally depends on the artist for me. If they are already super famous, or the albums old, no more than $10 (if it costs more than that I just buy the tracks I want most). If its an indie artist that I really like and don't want to see them disappear Ill just pay whatever they want, and...