Bumping this one up...
I just leased a 56lb cylinder from my local AirGas distributor... It's $70/year for the cylinder itself, and $95 to refill it. That works out to $1.69/lb for the nitrous itself, so it doesn't seem like it will take too long to make up for the $70 annual lease cost for the big bottle. Plus, there is a certain convenience factor in being able to fill my bottle myself at home or at the track.
I use a cheap Harbor Freight digital scale to weigh the bottle while filling, and as long as the small bottle is pretty cold, there's no problem getting a full 2lbs. in there.