The simple computation rests on the price of gas, which, thankfully, has fallen a bit. But a more accurate computation would take into account the actual wear and tear on the auto as well.
For me, I drive a car which cost about $25,000, and I expect to use it for about 150,000 miles, and then sell it for about $2,000. That means on a per mile basis, it costs about $0.15. Fuel, at $2.45 per gallon and 45mpg, costs a little over $0.05 per mile. I’m intentionally not accounting for insurance or maintenance, because I do need a car, and even without a commute, I would incur these expenses. This calculation is merely the cost of driving each day to work.
Well, with a per-mile cost of $0.207, and a round trip of 85 miles, the shuttle saves me $17.66 per trip.