Depending on which bike you have, they can be obscenely expensive.
If the rotor is a bit scored and you want to put new pads on it, if the rotor is still above the minimum thickness you can put new pads on, and they will quickly wear in to small grooves or ridges on the rotor, without compromising your braking power too much.
This is one of those things where you need to have some experience to know when a rotor is still ok, or when its shot. If your brake pads wore down to bare metal, the rotors may be shot.
Last edited by KCW; 03-18-2018 at 09:06 AM.