I know a guy in Keller, TX that picked up a LOT for $10, we're talking maybe 15-20 lbs. What's the catch you say? Rock was infected with bubble algae. Still a good deal though, especially if he can get some crabs to get rid of it.
Cheap Live Rock can be hard to find, theres no definite source where you can go to get it, I was so lucky I got mine with my tank, 70 lbs of live rock, maybe more, with a fully functional 75 gallon with pumps and filters and skimmers all for $350, which would have been enough just to cover the live rock.
Shop around and try ----/your local classifieds, check the trading section of the board on here often.
Cheap, good quality live rock can be found, it just takes persistance.