Base rock is just plain old rock. People use this as a base for all their LR. It saves you money, and you can hide alot of it under the LR. You should be able to get it at your LFS, I got mine for a $1 per lb.
Our LFS sell them for 1.99/p but thats still a decent deal I guess. I always just get the live rock but if you really wanna push for the most rock for your money its the way to go. Make sure and get a little LR though or it will take a long time to change over.
go to ---- and search by seller name of HIROCKS , he has hawiian base rock when they widened a harbor, I got 60lb for about $65 shipped, 3 months later and there's now coraline spots all over it, give it another 6months and it will be covered