Well, there are two general categories for price. The economical price, anywhere from $150 to $400 depending on the size of the plotter. You'll find plotters from USCutter in this range. The other category would be the "professional" plotters, and the price will jump from $800-$4000+. I doubt you're looking to start a sign business so I'd probably try out an economical plotter. If you search for USCutter on ebay you can get some good info and the "going rate". I have one of these 12" plotters myself and gotta say it does it's job well. It comes with the software required to cut, however, the software is weak and difficult, so you'll probably still need to use something like Paint Shop to make your art plotter ready. I've been using mine for about 5 months, a few times a week, and haven't had any problems, haven't even changed the blades yet.
If you went with a professional plotter you would be able to do a LOT more stuff, including making your own signs on a large scale, etc. But if you're just looking for a cutter for vinyl on a disc, the 12" USCutter is perfect. If buy chance you do end up buying one, let me know. They can be tricky to set up in the beginning, and I may be able to help you. You'll also find 12" vinyl rolls and 12" trasfer tape rolls for less than $20 each on ebay. Oh, you'll also be labeled a "plotter cheater" because you don't cut things out by hand anymore!
Some cutting tips:
Cut designs on the slowest speed
Keep your vinyl dust free
Use transfer tape to apply your design to the disc
For jobs I've done using the plotter check out these pictures.