Interesting. I bought 2 cans of R-12 on ebay last friday. Total refrigerant weight, using my newly honed "weights an measures" skills is 1.5 pounds.
Total cost was $25 delivered, or $16.60 per pound.
I think I got lucky, but even so, it's a long way from your $100/lb figure.
I just don't see any reason today to convert an A/C system to something other than the refrigerant it was designed for.