We have an artificial "reef ball" reef here too ... The design we used was primarily to attract lobster (the American lobster with the claws), but lots of fishies and soft corals seem to like them too.

I don't know about cheaper materials. But it does seem anything that lands in the ocean, it doesn't take long before the sea takes it over. One of our best dive sites is this train bridge. The supports for the bridge are completely carpeted in anemones and such. Amazing . . .

Ron