First, the front pads are different than the rear pads. Second, the calipers therefore are not interchangeable between front/rear wheels.
PinWall Cyles (fleabay) usually has used calipers. I got a rear one for $53.98 delivered and installed it no problem about 14,000 miles ago. Do make sure you either anneal the banjo washers or get some new ones if you remove the caliper. If you upgrade you are looking at the high end of 3 figures. (for one caliper).
