In any case, R22, which is what Armstrong units use, is about 250 for 30 lbs.
I don't use R12 because inferior but adequate automotive cooling can be achieved for a lot less money with R134A, so I convert to R134.
Before R12 was banned, A LOT of it was made to be sold later. There was a price bump before R134 was common in what became "older cars", (do you follow me here?).
Once demand for R12 dropped, the price has declined from its highs.
All that said, while I convert cars, I would spring for R22 for an RV A/C,
The fact that I am opinionated does not presuppose that I am wrong......