From what I can tell it's about $8000 to have a compressed natural gas station adjacent to my garage. The Honda Civic NGV (natural gas vehicle) is at $26,000 msrp. With 7.8 gallons of CNG capacity the Civic NGV should have a range of 280 miles (at 36 mpg average).
Driving 15,000 miles per year at 30 mpg equals 500 gallons of gasoline per year. If I wanted the installation ($8000) to pay off after two years then ($8000 / 2 = $4000; $4000 / 500 gallons per year = $8 per gallon for gasoline) gasoline would need to be $8 per gallon. A four year payoff would require gasoline stay above $4 per gallon for 4 years.
Of course I haven't accounted for the cost of natural gas.
I wouldn't do it for the cost. I'd do it because gasoline funds Venezuela, Iran, and Lybia.