If your frame and fork have disc mounts, you can expect to pay about $300+ for a new set of wheels and a set of Avid BB7 brakes. If you want hydraulic disc brakes you're looking at closer to $500+. Only you can say if it's worth it or not. I mean, heck, I'm sure there are people out there who are going to buy the new SRAM XX stuff where the cassette alone costs over $300...