I did look into this a few years back. I think it was when gas went up to $2/gal. and everyone was freaking out. Yeh, $2 would be nice today. There was a CNG conversion for a '95 Cherokee on E-Bay at the time. It was a dual fuel unit. I think the Fed Gov bought some of these. You will see some Ford Contour, Ford Crown Vics, Chevy Caveliers, and some trucks on E-Bay with CNG.
One of the guys were I work worked on this stuff at a previous job. He said about a 10 precent loss in power and possibly fuel milage. I did the math, it it broke even at the time. There were only two filling stations in my state, both 1 hour away. So, if the infrastructure is there, then it might be feasable. Also, the reduced range. But if dual fuel, then you'd still be able to run gas.
From what I understand, if the motor is built for CNG, it can run higher compression and get as good or better mpg. Also, some do conversions. I want to say $1500 for the kit, then labor. Also, check as some of the propane kits are not certified, and are for Off Road. I don't know about the CNG kits though.
Tom