DIY DIY DIY! Porsche brakes are some of the easiest you will ever change.
Like RENNX said, I got mine (all four rotors, low dust front & rear pads and two brake wear sensors) for $450 at
www.pap-parts.com
MOST IMPORTANT! When you change the brakes, bleed the system completely changing out the fluid. The manual says you should do it every 2- years max. If you don't, you'll get condensation build-up and boiling brake fluid.
That's what's happening if you ever hit the brakes and they suddenly start to flutter and let go.
Why can't you turn/machine your rotors? I measured a brand new rotor and it only has 2mm more thickness than the minimum. With only 2mm of wear depth, there's not much room for machining them down. My first set of front rotors lasted 60K miles through two sets of pads with no problems and no machining. I had purchased a pair for the back, but they are still in great shape so we'll see how long they last.