A prius is not a good example because of the batterys etc. so may have a large drop in value beyond a certain age.
Your calculation is no good for a general case though. Assume a car depreciates a constant percentage each year then apply that to the purchase price of new vs used and you will get a better idea of the real situation.
i.e.
13k @ 12% year = ~6k loss (sell for 7k)
24k @ 12% year = ~11k loss (sell for 13k)
