it is a liability issue. You need to stress to the owner that certain things need to be replaced for safety's sake. There are aftermarket choices you can make that do not require $300/rotor, but you probably will be spending $100+ for a single rotor.
No one is likely going to surface your rotor because of the liability issue. This is a time when the owner needs to 'bite the bullet' and pony-up the $ for his safety. The factory manual will tell you what the min. Thickness is. You can decide if the clean-up of the rotor would put thickness below that. Sometimes the min. Thickness is cast-into the rotor itself, try there, then the factory manual. Really, braking components are no-place to scrimp on your budget. I don't even rebuild master cylinders any more, i just buy new ones. $100+ for a new one vs. ~$25 for a kit, which may or may-not work? How-much do you value your life-at for a $ figure?