That engine MSRP was $249.95. That model has been around since the beginning, and was equipped with the early 5 volt boards as well as the later 3 volt boards. It's resale value should be better if it is the later 3 volt version.
If someone with DCS could check the mileage, that would be helpful. Regardless, if it is in excellent condition, my estimate(as others have offered) would be about 50% of the original retail.