There are no substitutes for live rock, but I have been successful "cloning" some.
It takes time and patience, but you can get alot more good quality live rock for alot less money.
I got some good uncured live rock, about 30% of what I actually needed finished. I mean the stuff with the most tube worms and algae that you can imagine. I choose "uncured" because most of the curing process is killing off stuff and I wanted everything I could try to cultivate. I then went to a store that sells rocks and stone for yard use and picked out "holey limestone" pieces that would look nice. Avoid rocks that look cut or have straight edges-the more organic the better. Wash it REAL good.
I used this limestone as the base rock and put the purchased live rock on top and went through the curing process. After 6 months you couldn't tell the difference between the two types.
The key to live rock is 2 things: The live bacteria and animals that inhabit it, and the high surface area in the stone itself for them to live.