A new, filled, 20# tank from a gas distributor will probably run $150.
Price depends on where you live, how many home brewers are losing their jobs or moving
Patience saved me a bunch. Or cost me a bunch as I now keep three cylinders in rotation.
I've purchased a 50# cylinder for $35, and a 20# for $20. I swapped the 50# for a 20#.
I paid $150 to a young man going back to college, for a 10# aluminum tank and a bunch of brewing supplies. I sold the brewing supplies for $150.
I got an out of date steel 20# tank for free from a guy who'd moved and lost his keg emptying buddies. That cost me about a gallon of gasoline.
Call your local beverage (beer mostly) distributor and ask them for prices on a 20# if you have room for the height. Beer and pop distributors deal in large quantities of 20# cylinders for pop dispensers in gas stations and small stores.