Great question!
Using an '06 with 40k as a baseline for $20,000. For each year up or down I'd move that by $1,000. Also, $1,000 for each 10,000 miles. So, a '07 with 30k, would be $22,000 in this example.
An '04 with 60K would be $16,000.
Of course condition, pedigree and distance from MD can...