With big tanks it is a very good idea to use very little "live rock". Buying it at 7 bucks a lb is obviously expensive. You can buy some amazing dry rock from Bulk Reef Supply. You can even request certain sized pieces. In addition to getting big pieces from them you can fill in with any old base rock. There are frequent people on Craigslist and reef sites selling off rock as well for significantly cheaper. Obviously the dry rock needs to cure and become live rock, but that is part of the patience involved with setting up a quality large tank. I personally got all of my live rock(400+ lbs. total) from a few different people on Craigslist and only paid on average approximately $1.50 a lb.