If you visually inspect your rotors, and see ridges and valleys on the surface of the rotor and also if you can feel these as well (remember that there are two sides of the rotor) then the rule of thumb is that you "should" resurface the rotor or replace it (ie: drilled or slotted rotors cannot be resurfaced). Thus you would or "should" replace or resurface both fronts or both rears as the case may be.

Now, if your rotors are clear and not blemished with ridges or valleys then you probably do not need to resurface them.

The second part of your question is... "Is $150 per set too expensive to resurface rotors and replace pads?" This would probably be a 2.5 to 3.0 hour job per set plus machine time and cutting tools. So three hours times the going rate per hour probably gets you about $150 per set with rotor turning.