I would not recommend that you spend the money to add a bath to an unfinished basement. It's a nice convenience but will probably not add to the value of the home. When I have taken buyers to see a place that is listed as having two baths and the second bath is in an unfinished basement they usually feel deceived. Your listing agent could suggest to potential buyers that a second bath could be added in the basement or, for more money, on the second floor. It would be better to lower your price by the cost of the basement bath.
Ellen G. Friedman, Keller Williams Realty, 617-448-1542 or firstname.lastname@example.org