I can't even guess how much it would cost, that would take an on-site evaluation of a number of factors that would affect the final price. A floor drain fixture is something like 12" deep, and it would have to be connected to the main sewer drain. The connecting pipe would have to have at least a 1/4" per foot slope to where is accessed the sewer. So, the distance to the sewerline, and its depth would be at least two major concerns. It would be a very labor intensive job to retrofit the drain which would have been dirt cheap during construction.