First of all we love questions.

For your problem. here is my suggestion.

define variables:

P: current car price. O: original car price.

miles: mile record of the car. months: age of the car.

10,000 miles each year, driven 36 months, that is 30,000miles in total. And the price difference is $9,000. So the car lost $0.3 for its each mile to go. Then we can have a price function according to the miles:

P(miles) = O - 0.3*miles (miles<66,666)

I assume the P is also propotional to (1/months), so here is a price function according to months as well:

P(months) = O * (19.8/months) (months>19.8)

Finally, the average price P = (P(miles) + P(months))/2

Actually you can choose the price which the most benifits to you. This is totally up to you.