First of all, if you needed pads and rotors, they should be for the front. The rears rarely warp and don't wear a whole lot, simply because the fronts do most of the work. As for the price, sounds real high. Here are some no-research-done prices, so you could probably even find lower!
Brake Master Cylinder - $80
Metal Master Rear Pads - $32.47
KVR Slotted or Cross-Drilled Rotors - $78.30 ea.
Hmmm... that puts you at a whopping $269.07 plus a little for shipping. Still plenty less than 300 bucks. And that's for performance brakes... I doubt that is what they were quoting you on. If it were up to me? I'd make the effort to do it myself. I think that's worth $180 or so.
Nathan