Well quicky example. I picked up a set of J30 Calipers to replace my 180sx ones @ a wreckers for $40.00 (didnt need to be rebuilt by any means) but I did anyway since I ended up powder coating them and the seals/bleeders were a few bucks.
So I got brand new calipers for about $50.
Rebuilding calipers is simple btw. As in a few minutes and your done.