I actually just started sculpting myself about 2 weeks ago and as Jeric said it is more costly then buying merch as well as time consuming (so much sanding) materials cost me between $100-200. The good things with Super Sculpy or Fimo is you can keep working with it you can usually just knead and blend it back and start over, and even when its baked you can still shape it a bit by sanding it and even if you sand it too much you can rough up the area a bit and add more clay then bake it so its forgiving to a point.
I'm really having a fun time sculpting and have some really cool ideas I'd like to try in the future once I'm a little more comfortable in my abilities. I think Michaels does sell sculpting clay at least on their website they do. I do suggest reading up on some tutorials to know what your getting what into, what materials you might need and to also learn a few tips and tricks, I spent the better part of a week doing that while waiting for supplies to get here.
I'm working on two right now Twilight my first sculpture and DustyKatt's OC (from SBMF) they are about 75% done I can't wait to finish it them up and start painting.