I paid $35 a pair for S10 rear springs at a junkyard - that even included them torching them off rather than me trying to break old spring bolts free. Worked great for two years of abuse until I broke a spring. I recently got new lift springs up front and the ride is so much better (and my Jeep is a trail only Jeep) in the front than the back that I am going to be buying the same springs for the rear now.
The springs are the 3.5" Fabtech springs from Amazon - only $65 each, but they do not come with bushings so about $100 a corner.
1994 Jeep YJ off road toy
1981 CJ7 SBC 350
2005 KJ Libertyt 4x4 CRD w/ 2.5" Frankenlift DD