Going rate for iPhone Icon?

I've done some iOS development and have some niche apps already in the app store, but this latest app that i'm working on I think will appeal to a larger audience and could be useful to people. In the past I've worked with a friend who does some design on the side, but this really isnt their main focus. I would like to get an idea of what to expect if I were to work with some of the amazingly talented designers on here. I'm just a single indie developer so budget is tight...