Dear Brookline residence,
It all depends if are allowed by town's zoning regulations to add the basement to your living area or not.
If permitted and you can added to your public record and eventually to your deed, then you have a golden egg here to hatch. In the other hand if by zoning law you are not allowed to do it, then you may have an finished space which may appeal to some buyers, as bonus, but not willing to pay a much higher price for it, and you would not be able to listed on your marketing material as extra square feet of space.
I don't know the size of your basement and the level of finish you are planing to do, but do not over spend.
I suggest you consult an attorney regarding the zoning regulations, you may be able to get variance.
Please let me know if i can help.