When is the best point to buy a car?

Sorry if this has been posted before, but I see discussions all the time about the merits of buying a new or used car. If you could look at the total cost of a car, add in repairs, and divide by the number of years you could expect to own that car, at what point do you think you could buy the car and and keep the "cost per year" as low as possible. For me, it almost seems buying a car with around 100k miles on it, and paying less than 10 k would be pretty cheap.