Annual Insurance = I
Miles travelled on one Gallon of petrol = G
cost of one gallon of Petrol = P
average annual Maintenance costs = M
annual Tax (of whatever sort americans pay) = T
average monthly cost of assorted Expenses (parking tickets, toll charges, etc) = E
average Number of miles driven per year = N
Cost per mile = C
--If there are any other costs, or if the car brings any income in (such as a weekend job delivering papers around your neighbourhood) or if any of the above variables are hard to calculate, let me know and I'll try to figure it out--
[12E+I+M+T+(N*P/G))]/N = C
I'm a bit shaky when it comes to maths, however, and looking at the formula I've just figured out completely terrifies me, so you may want to get someone intelligent to check the logic.
PS: The above formula does involve for the initial cost of the car, or any loan perhaps taken out to cover the cost thereof. Happy Driving!