bulkreefsupply can supply decently priced dry rock. It takes longer to get an aquarium up and going with it but saves a decent amount of money. Just look at reviews on the different kinds some have had phosphate leaching with specific types from them.
The term "LIVE ROCK" only means it's been wet long enough to have aerobic bacteria growing on it. It has nothing to due with the composition of the rock itself.
You can pretty much put in any rock you want as long as you understand how that rock/mineral will impact your water chemsity.
I've been using lava rock for a little while now. I think it looks nicer in the display than LR, and I haven't noticed a change in water chemistry.
Although I still use plenty of LR down in the filter