I am using 1.5 to 2 lbs per a gallon it seems these days. I have between 15-20 lbs in my 10 gallons and about 25 in my 20 and 24 gallon tanks. I am using in the neighborhood of 50 lbs in my 55 gallon tank.
I am using Tahitian Moon in both the 20 and 24 gallon tanks. In both of the 10 gallons and the 55 gallon I am using Pavestone Pulverized Limestone. This stuff is calcium based and is about 6 bucks per a 50 lbs bag. It seems to be working just as well as the specialty sands sold at the LFS and doing so for a fraction of the price.
In all fairness with 5 tanks from 10-55 gallons I have a whole two fish. Most of my tanks feature a considerable amount of live rock and corals for the most part.
Decide what fish and inverts your going to have, then set the tank up to meet thier needs using the style of sand thats most appropriate and at the proper depth required. Then there is the filtration aspect of it...ie, Deep Sand Bed or something thats shallower or BB if appropriate. It all comes back to the fact that more times than not you are creating a certian biotope. If pristine water and SPS corals are the goal a BB might be the best way to go. Then again if you are set on keeping a Jawfish then a DSB might be the best direction to look towards. A sand sifting Goby will do better in a finer grade sand bed than one of Puka Shell or Crushed Coral. Again success will best be arrived at with a adequate plan so research your critters needs.
If you need to get a rough idea of how much sand you will need to achieve a specific depth then look at
www.garf.org. Go to the left hand menu and select calculators, then select sand bed depth from the drop down menu. Plug in the demensions of your tank and the depth of sand desired and it will give you a rough guestimate of how much sand you need to buy.