100 pounds for a good 1-1 1/4" layer... might even get you a little more. I tend to splurge on substrates and go ahead and get the brands from the lfs.... so I doubt you really wanna hear what I have to say about that, because I usually go with Caribsea Tahitian moon, sugar fine white, or aragonite, depending on what kind of fish are going in there. I would be spending $50-150 on that tank for substrate.
50 lbs would do to cover the bottom, 150 would be a nice, really thick layer, 100 pounds is probably ideal, kinda depending on what kind of sand you're using(density).