I have been seeing them for 35 or so used and cleaned. I think I saw a deal on a site that had 4 kegs for $115. I will look and see if I can find the site for you.
I would go with pedalbiker. He sells them on here for cheaper than his site linked below. Sells them at $25 a piece or you can buy ones with loose rubber bottoms or tops that can be glued on for even less. I have bought all my kegs from him, as well as many others around here. There is a very long thread that discusses them but I can't find it now.......