Slate or granite is another route to go. I can get a 12 x 12 inch slate tiles at the local hardware for 1.50$ each (granite tiles are about 4-5$ i think?). The tiles are really easy to break into pieces with a hammer or gravity, and with a stick of epoxy you can assemble the pieces however you like! PVC
tubing is also pretty cheap if your livestock like "caves"; gives you a frame for rocks, rubble, plants, etc.