Really? I actually prefer the bump mapping tools in GIMP! It's one of about five things that I think it does better -- the PS bump maps always come out "shiny" no matter how much I smooth out and gaussian blur my alpha, or what settings I use.
That said, I got CS5 Extended through my college program for a little over $200. I do like it better than GIMP -- I've found that when I'm experimenting with filters or trying to do something a little funny, I tend to be able to get the effect I want a lot more easily in Photoshop. Fewer buttons pressed for the same effects, smarter auto-correct and fill tools, the "stroke" effects in PS always seem to come out a lot better, and, most importantly (for me), when I find myself confuzzled, there's tons of support on the web for PS users, and shiploads of tutorals. GIMP... not so much. Also, the 3D tools in CS4+ Extended are to die for. I remember I used to get so, so angry trying to make the straps on dresses straight, or panties that cut across the butt not have bizarre jagged edges, and while Photoshop can't do much to make up for the awful UV mapping that happens in some places, in many cases having a 3D painting option built in is a lifesaver.
That said, GIMP is an amazingly versatile tool, especially for the price.
To me, Photoshop is worth $200 more than GIMP... but not $800 more.
If I hadn't been able to get student pricing, I'd probably still be using GIMP... at least until I turned a profit in SL and felt like it was worth the investment.