LPG rigs can be found just about anywhere for sell cheap. Flea markets, garage sells, want ads. I bet you could check a number of companies that sell propane on the local market and find 5 or 6 complete used rigs for sell. Especially in rural communities with agrigculture equipement.

I have seen them go for 1000 but then again most seem to be going for 250 or less.

I use to run a 100 gallon tank LPG system on my old pickup back in the 70s and early 80s. I think LPG was about 10 MPG but cost about $0.40 a gallon. I had not modified the engine to high compression or changed the timing because I ran it on gasoline and LPG. One plus for LPG is the engine will last forever! So will sparkplugs which even with the cheap plugs back then would last 90000 miles.

Also the engine oil would last forever(figuratively speaking)The engine runs so clean that when I changed the oil at 5000 miles it still looked new with very little discoloration.

Of course the downside is it is kinda a dangerous fuel to use and your system has to be maintained in perfect condition.