There are two ways to answer that question:
1. Miles per gallon - First of all, CNG is a gas not a liquid so the term gallon does not really have any meaning. The correct term is Gasoline Gallon Equivalent, or GGE, see link:
http://en.wikipedia.org/wiki/GGE
CNG's optimum compression ratio is about 15:1, much higher than gasoline. Since my vehicles are bi-fuel they are tuned to run on the compression ratio of gasoline (8:1 or 9:1). I therefore have a slight reduction in power and torque when on CNG. The mileage is about the same at highway speeds, but in stop and go traffic and city traffic my mileage is less. Here are my estimates on mileage:
Cavalier - Gasoline: 80% highway and 20% city - 30 MPG
CNG: 80% highway and 20% city - 28 - 29 MPGGE
Silverado - Gasoline: 50% highway and 50% city - 14 MPG
CNG: 50% highway and 50% city - 12 MPGGE
2. Cost per mile - This is dependent on the price of gasoline versus the price of CNG. Gasoline prices changes almost everday and depending on how the CEO of OPEC feels it goes up or down. CNG is much different. It is an American fuel and, at least in Utah, takes an act of the Public Service Committee to change the price. Right now gasoline is at $3.67/gal and CNG at $1.27/gal. So, lets take my Cavalier at 30 MPG and 28.5 MPGGE for example. On gasoline it costs me $0.12 per mile. On CNG it costs me $0.04 per mile. That is a savings of $0.08 per mile. So while on CNG, my gasoline equivalent mileage is ~87 MPG.
