I just bought another new 320 cu. ft tank a few months ago for $350. Don't know what to call it other than 320 cu. ft., it's 56" to the top of the cap and about 8" OD. Watch Craig's List for used tanks, they become available fairly often around here. If buying a used tank, check the certification date stamped on the side of the tank near the top. They have to be pressure tested and re-certified every 10 years.
It costs me $24 and change to get my O2 refilled. Make sure you have a place fairly nearby that'll refill tanks rather than exchange them if you buy one. You don't want to take a tank you just had re-certified and exchange it for who knows what.
The second tank I bought is for C-25, I don't breath that stuff. My O2 tank I had to have re-certified a couple years for $80 included a new valve.
All in all, I'd say that owning your own tank is a less expensive option in the long run, especially as how, unfortunately, your probably gonna use it for the rest of your life anyway.