So what's your current bike?
I put a disc fork on my Kona Jake, and I'm very happy with it. I already had a disc wheel and brake, but I'd guess that figuring in the cost of those it could have come in around $350. If you like your bike except for the tires and brakes, then go ahead and convert. I highly recommend Avid BB7 brakes, and Avid SpeedDial levers.