You’re buying 12oz cans? I have no idea what they go for, but I do recall seeing the “magic” DIY cans at a store for $20-$25. Crazy.
Don’t you keep a 30lb cylinder around? Last summer I bought 5 of them at $89 each. At that price you could hang around the Autozone parking lot with a set of gauges and make a killing doing on the spot recharges.