I think you'd have to import one of the systems from Europe. Even cheaper cars there are commonly converted to dual fuel. Propane is freezing at lower temperatures, so gas start if often a must in the winter. Better systems have computer switching over at certain temperature/speed, while cheaper systems have manual switch.
YM compared the propane prices at local hardware stories. At truck stops in CA it cost now $3.29 v/s 4.09 for premium.
Flying J: Fuel Prices > Gasoline & Car/RV Diesel Fuel Prices Propane Pricing
If you use propane for house heating and have it delivered to your home in bulk the price is even lower.
Still it is economical stretch, especially when you compare how cheap MB diesels sell.