I originally bought an 80cu ft Argon tank filled for $143.27 (would cost $23.00 for refill). That small tank seemed to go fast, so when it was gone, I went back and they allowed the full original purchase price (minus the cost of the Argon $23.00) towards a 125cu ft, which they sell filled with Argon for $157.18. I don't know the refill price.
For my usage so far, the 125 size seems fine.