R-12 Cost/Value

This is more out of curiosity than anything else. I just had a friend
purge and refill my AC on the 92 with R-12. He has seven twenty pound
cannisters of the R-12 that he bought many years ago when they started
to phase it out. I was thinking about trying to buy one from him for
the local Miata guys that are still running R-12 but have no idea how
much one of these is worth these days. I do know I wish I had thought
of stockpiling it myself. Any ideas on what a fair price would be for
one? Thanks.

Tom
92 Red