You can buy base rock very cheap and given time it will become live rock. You just need a few peices of LR to have the coraline and macros so that they will spread over to the base rock. If conditions are right it can occur 4 to 6 mounths.
I used only 50% LR on my last tank and 50% base and put the LR on top and now in the places where the base shows you can't tell the difference between it and the origonal LR.
If you use a different type of rock(slate, quartz, or others that look like thay my never have been in the ocean) the process may take a little longer but will occur. HTH