That's an absolutely crappy deal!
$150. for a 40? Ridiculous!!
Go onto ebay and buy an 80 from Indiana for about $80. shipped for free.
I'll go find you a link in a few minutes to where you can buy-it-now for the price and have it in two or three days. I'll vouch for the seller until the moon crashes into Earth - he's reliable, accurate, and fast.
edit: Well, well, the price of steel, I guess. Nevertheless this is a link to an 80 gal. tank for C25 for $144. delivered to your door. If you look around the seller's auctions you'll find that he will sell you the size you posted for $94. So still a MUCH better deal than in Northern, but not as good as the last time I bought a bottle from him.
There's no way to accurately predict your usage as it depends on severa variables.
BTW, when you own a tank you still do the exchange thing. Take yours in, they'll give you another one. You'll likely never see your original again. The good thing about owning tanks is that you don't have to pay anyone's tank rental fee.
http://cgi.ebay.com/80-CF-WELDING-CY...QQcmdZViewItem
Sheesh, I hadn't seen your post about the lease. How much a month, if anything?