$15,885 option to upgrade to CNG in a new Chevy Express.
Is that in Yen? You'd have to do some math to figure if there would ever be a payback from that kind of investment.
We used to run propane in some of our trucks in the eighties. It was about a $1,500 conversion including the belly tank, but we only had to replace the carburetor and run lines to the tank. If the tank was overfull the lines would ice and the truck wouldn't start. The only reason we went away from propane was that the price of gas dropped and we stopped using slide-ins and propane heaters.
Propane doesn't have the bang that gas does so your mileage will be less, but it burns very cleanly so there are no carbon gunk issues. Your plugs will look like new when it's time to replace them.
PS: We ran the Onan slide-in engine on propane too, which was a significant benefit since they were prone to having carbon build-up issues.