if you put pads that are resurfaced with 80% left then you will have 20% less time on the rotors before you may have to resurface them.
May is the operative word, but probably not since rotor wear isn't a function of brake pad wear. Rotors wear because they contact the brake pad, brake pads wear because they contact the rotor. A rotor doesn't wear any less contacting new brake pads than they wear contacting 80% brake pads. In fact, they wear more.
it most cost 50 more but its best to put new pads on with new rotors. probably safest too.
So you should replace pads at 80% pad life because its unsafe...which would be about every two or three months? I didn't know that.