“Propane does tend to be more expensive than CNG but in most location less expensive than gasoline on a per energy basis.”
CNG is indeed less expensive. But, as soon as CNG vehicles begin to make a presence expect to see road taxes to be tacked on to the price. Btw, I’m old enough to remember when diesel price was about half of gasoline. Also, while CNG has some advantages such as price (for the time being) and a cleaner burn, it has some disadvantages such as less miles per gallon and horsepower per equivalent engine. So, those who haul or pull heavy loads may not be too eager to make the switch.
Much of the added price in diesel came from the EPA Ultra-Low Sulfur Diesel regulations. While diesel has a little more tax, the processing of it in the refinery became much more expensive.