Is 75 dollars ea. side reasonable labor for installing front rotor backing plates on 2002 Hyundai Elantra when all 3 the rotors pads calipers combined install labor is only 75 dollars ea. side?

Actually, yes it is. Wether you replace all three parts or just the rotor backing plates, you still need to dissasemble and re-assemble all of the same parts. Basicly it's just a difference of putting back the used parts or installing the new ones. either way, same amount of work. Hope this helps.