I am surprised no one mentioned edmunds.com with their TMV (True Market Value). Go to their main page and click on TMV near the top left click on True Market Value – under certified cars and used cars. I just picked the MDX for illustrative purposes and picked 2008 year. A pop up window lets you pick year, make and model.

Brings you to this page: http://www.edmunds.com/acura/mdx/2008/index.html

Then select the trim package

Brings you to, which gives dealer trade-in, private party sale, and retail sale
http://www.edmunds.com/used/2008/acura/mdx/100933709/prices.html

you can also click on customize appraisal for specifics for each individual car and go by zipcode too.