Welding gas consumption

[67mater]

I rented and filled a very large gas tank when I borrowed my uncle's MIG. I don't know off hand how big, but it's probably 4' tall and 10" in diameter. Annyways, it was around 150lbs of pressure when I bought it. I didn't change any settings on the regulator as I get great welds out of it. When I was done welding the cab (never leaving the tank valve open over night or anything; it is an old mig and likely leaks) I was all the way down to 80lbs. Is that reasonable? What I was seeign was a few lbs drop over night with valve shut.

I went to use it today and it's at 25lbs. VALVE SHUT!!! what gives? I tried soapy water on everything but see no hint of leak (tank valve, regulator, hose etc.) I didn't check the pressure until after I had done about an hour of spot welding, but how fast will a large tank run out? is it conceavable that the tank should drop 10 lbs eveytime I really use it? and any shot of complaining to the shop I got the gas from? or do I have to spend $300 more to fill it again? (which I won't as I have a flux core for the little bit of body welding I have left right now)