Use base rockand top it off with some live rock to "seed it". It's both cheaper, and more "eco friendly". I used this product: www.marcorocks.com
Here's what 50 lbs looks like. Very porous, very nice rock.
I've had it in my sump now for about a month, in preparation for use in my new clam tank and new seahorse tank.
It's now FILLED with munnid isopods, mysid, gammarus amphipods, and yeah...even baby bristle worms. It has JUST begun to color up with a dark purple, crimson coralline.
Dave