For aquarium sand, you're looking at up to $1/pound. The most expensive ones are the black tahitian moon sand, with most of the others running somewhat less. It's still the most expensive option.
For general purpose sand, like what you get from a hardware store, it can be about $0.10/pound and sometimes a bit less.
Pool filter sand runs somewhere in the middle.
I think the general rule of thumb for a starting place is ~1 pound/US gallon. It is going to depend on how deep you want it and the shape of your tank though.