Unfinished it adds nothing.
Finished it add value but not as much as the above grade space does. As an example; if homes of your like kind are selling for $200 per square foot than the below grade space may be worth perhaps only half of that. These prices are only a presumption and need to be compared to actual experience in your area.
All the best,
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
My suggestion would be for several local Realtors to give you a market evaluation. This should be free and it will give you a range of value for your home compared to other homes that recently sold.