Propane is mostly a byproduct of refining crude oil and is generally tied to oil prices, where natural gas is directly obtained from the wells and has gone through tremendous improvements in technology and is prices about 30% of what it was a few years ago. LNG and CNG are both the same molocules, but just at different compressions. Compressing it to a liquid increases the density (storage capacity), but increases the costs. I don't think it is certain which one of these will win out at this point. I really think either is much better than electric. These have a normal range.