I had a somewhat similar problem with the rear brakes and a caliper going south on me. After looking into the replacement costs AND considering whether I really want to swap in a "used" brake part........I started saving for a new total brake swap. I had a lot of miles and figured that other stuff would be going bad soon too (not to mention that improved safety was a positive too).
Not that you want to go this route, but here's what I did: $800 got me the 2004 Cobra brake kit front and rear with all parts and even included stainless lines (calipers, discs, pads, and everything else needed). I kept the stock GT master cylinder.
Of course, you could go to a junkyard and get a rear caliper for cheap.
Cobra brakes were one of my favorite upgrades!