adobe photoshop or illustrator are probably the best programs to use on a mac, they are super good on them.
as for price, you can just *couch*torrent*cough* it. I had a super easy keygen to use, i might be able to dig it up for you
If you don't want to torrent it and want something cheap you could try Gimp. I don't know if it is for Mac but I would assume so. It is like a slightly more simplified version of Photoshop for free since it is opensource.