I don't have a deep sandbed, but used about 100 lbs for my 75g bowfront.
I would think if you wanted a deep sandbed, you would need to at least double that.
Just in case you hadn't thought about it...and because I prefer the black sand....you can get the black sand at George Townsend in OKC (it's in the phonebook.) He has 2 different sizes. The smaller is more uniformly black. The larger has more "garbage" in it i.e. larger, playsand colored pieces. I have found that I bring in, and pieces chip off of my live rock, anyway, so I went with the large size. It was approx 12.00 for 100 lbs.
Of course it's not live, but it will cycle along with your rocks and tank, and be live by the time you add fish. It was very clean. I actually poured it straight out of the bag, into the tank, and had much less clouding than when I have used the "live" sand out of a bag.