The difference in value of properties w/ or w/o a basement may have some fluctuation based upon the volume of buyers in the market and inventory levels.
In a tight inventory with many buyers, those buyers may compromise on their list of "wants" and the differential between home w/ and w/o basements may carry an average figure as supported by recent comp sales. Contrast to a cycle with strong inventory and fewer buyers, you are looking to capture the attention of a single buyer who might prefer a basement. In order to swing the pendelum to your favor, the discounted price, or savings the buyer gains has to be a strong enough incentive. Refer to recent comps to evaluate current data with the help of a local Realtor.
There will always be buyers who have basements on a list of must haves.