Depends on the deal and the store. Prices can range from 5-9 dollars a lb. It also depends on how much you get in one sitting. Usually stores cut you a deal with 25 lbs and up.
BTW Unreefsafe usually means that a fish/invertabret will eat whatever corals you happen to buy, or will hunt your precious shrimp and crabs or the other way around shrimp hunting fish. THe larger your tank is the less of a factor this is. Most life that you buy for your tank will nibble on the rock regardless because the rock contains microalgae after a time. What you DO NOT WANT is something that eats coraline algae, ie purple stuff. Thats bad.