Last time I ran the numbers the US highway system costs around 3$/gallon of gas used on the highway system. So, double your gas costs, then add all the other costs and you can approximate the "net" cost of your commute.

It's more than your gas cost, though - there are other things like maintenance, amortized cost of car ownership, insurance, etc.

You don't just pay for gas, you pay for the car to put it in.