The cost of nitrous depends on what the "going rate" in your area is. Normal price here is $10 per pound
I pay $5 a pound and that is at someone's cost.
Some places in the lower 48 charge as low as like $2.50lb, but it just varies.
its about $4 a pound here.. you only have to fully fill it once..
when you get the tank, its empty..
after the pressure drops to the point where it is no longer effective, you fill it.. the tank isnt empty at that point. you may have a pound or 2 in there yet.. depends on your setup.. bottle heater etc...