supply and demand.. who really knows how pricing is determined? then again it depends on where you stop for propane.
I can remember on our Alaska trip I purchased 4 tanks of propane and pricing was all over the map
paid as little as 30 dollar Canadian to 50 dollars Canadian, don't remember it being pricy in Alaska either
but 30 dollars Canadian is about 22 USD. I've paid almost 40 USD too, for the same thing.
Personally I think they price it at what they feel like selling it.