The initial savings are apparent the first time you top off the tank. I commute from Norman to north Oklahoma City every weekday, and when I was driving the Impala, I was dropping $200 to $250 a month on gas alone. Going CNG saves me as much as $210 a month, since weekly fillups for the GX rarely top $8. I’m also getting much better gas mileage with the Civic, which also helps (36 MPG compared to the Impala’s 24ish). Doing quick math, my wife and I are going from spending $3,000 a year on fuel to about $390. In about three years, the price difference for the GX is pretty much covered.

Edit: As readers have pointed out, there are incentives in the form of rebates and tax credits for going CNG. The state of Oklahoma offers these tax incentives, and the federal government offers these incentives.

Also, in Oklahoma, Oklahoma Natural Gas offers a $1,000 rebate for the purchase of a CNG vehicle, though the rebate availability hinges on the availability of funds. I've personally submitted my paperwork for the rebate, so we'll see how long it takes to get to me.

Edit: I spoke with Michele Williams, fleet manager and alternative fuels specialist for Don Carlton Honda in Tulsa, who clarified some things about my GX to LX model and price comparison. Williams pointed out that it's more fair to compare the GX to the EX, which has an MSRP of $20,655. The reason this comparison is more fair is because both models come equipped with alloy wheels and security system. The difference in price is roughly $7,000.