I'm with renogaw; base rock is a great way to get more for less. I personally use about 30% live Fiji rock and the rest is all base in both the tanks I've setup, and will do the same with the next one. Within a few months you won't be able to tell the base rock from the live rock, and the base rock will become "live", considering it will have coraline algea growth, bacteria buildup, and critters growing in and on it. It's also about 1/4 of the cost of live rock in most places. In fact, at my LFS, I can get a 50lb box of base rock for $45. But most places sell it at about $2/lb for just loose pieces. As far as the amount, I'd get a total of about 150lbs of combined live and base rock to start. Keep in mind, you could potentially add coral that's already attached to rocks, so your overall weight will go up over time anyway.