You would need new wheels also.

Avid BB7s run about 130 bucks for the front and rear levers and cables. Wheels are about the same for the most basic disc wheels all the way up to what ever you want to spend.

You might want to look at Avid Single Digit 7 V brakes. That would be a huge upgrade from what I would assume is on there now and you wouldn't have to replace the wheels. They run about 40 bucks for the brakes and another 20 for the levers.