Where are you located? Most of my LFS
sell for $6-9 lb, but I see it listed on Craigslist for half that. If that is not an option, I would buy regular "dead" rock and then try to get a couple pieces of rubble with coralline. That will eventually spread. My LFS
sells small pieces (golf ball sized ) that break off their big stuff for $1 each. $12 is ridiculous.