Whether a system is cheap depends on a lot of factors besides the cost of the equipment. Let's pretend that your vehicle gets 15 mpg and you drive 30,000 miles/year. Diesel fuel will cost you $8,000/year @ $4/gallon and $10,000/year @ $5/gallon.

If you were to install a $6000 CNG system that substitutes 40% diesel and CNG costs $1.50/GGE ($1.18/GGE net), the payback on the CNG system will be about 2.8 years. If diesel goes up to $5.00/gallon, the payback becomes 2.1 years. If you do some dyno tuning with $4/gallon diesel and can now substitute 60% diesel, your payback becomes 1.9 years.

Instead of buying your CNG in Wartburg, let's say you live in Miami, OK where CNG costs $0.75/GGE ($0.43/GGE net) and you're substituting 50% of $4/gallon diesel, the payback is about 1.7 years. If diesel goes up to $5/gallon, the payback becomes 1.3 years.

Depending upon fuel prices, fuel consumption, annual mileage, installation cost, and substitution rate, a CNG system could actually be cheap.