If you want the cheap shit the mechanics use, you can do the job for $30. Probably $15 for pads and $15 to turn the rotors at your local machine shop.

Ideally, you want new pads to bed with new rotors....although IMO, it isnt the biggest deal in the world. Take a look at your rotors, see if they have any grooves in them and if theyre in good condition (which they may not be if the wear indicator on the pad has been making contact for a while). If theyre ok, dont worry about resurfacing them, just slap some pads on and call it a day. If you want to replace them, its $20 a rotor at autozone, and use the pads of your choice. For rear pads, its not going to make much of a difference.

In terms of which is cheaper...I dont know, ask the mechanic how much hes charging for labor and parts.