I cant say what the conversion would cost, but along with the cleaner fuel, you can make serious HP with natural gas. I saw a program that included a drag racer that uses N/G and the internal parts are like brand new after a season of raceing, and he runs I think its 14:1 compression, cant remember how fast he was going but he was hauling ass.
That is a costly conversion. CNG has less btu's than propane. It will burn cleaner and you will be able to eat out of your engine after 100k miles! For a good CNG motor you need about 12-1 static CR. You motor will last a very long time as cng has no carbon or VERY little, so the motor oil never discolors but should still be changed. More and more places sell cng but they a still few and far between where I live.
oooppss...Forgot to mention,fuel mileage would be less than with gasoline,unless dramatic build up for the loss in btu and the gain in octane were optimized, because you consume btu's when you drive. Cold weather start up is awesome,driveability improves, horsepower is down by 20% or so. I had a 302 Propane powered 1979 Ford Truck that I put 107k miles on before repowering with a 460 built up for propane use. Only downside is initial cost that takes forever to recover,loss of cargo space,and difficulty fueling when traveling.