"Value" is a word that can mean many different things to different groups of people.
A basement to one person could be of more value than to someone who prefers a slab. In Charlotte, we have many people from all over the country coming to live. Some are used to living with basements and request listings that have them, while some that are not, ask for listings on houses without basements. So "Value" is all in the eye of the beholder.
I would contact at least 3 agents to provide you with a comparason report. Preferably broken down to show homes with basements, homes without and an average of the two. Its a little more work for the agent, but maybe when, and if you sell, you'll remember that agents name who did the extra work for you and call them to assist you.
Hope that helps!
Wilkinson & Associates