I have a boat load of dead fiji rock for 2 dollars a pound. up to 100 pounds if you need that much.
I also have some tonga branch for 3 dollars a pound. [email protected]
You could also use tufa rock which may be available at LFS. The uncarved pieces are usually under $2/lb. The carved ones can be pricey though. I've also used lace rock in my tank as well, it's pretty dense.
CaribSea produces dry rock labeled "Reef Rocks", available in 30lb and 50lb boxes. Most LFS's can get it for you though some may not realize it is available from their distributor. Pricing should be around $1/lb. Some of the sizes and shapes are interesting, but you will want to pick out the best of the lot.