2. The amount, if any, which will compensate such displaced person for any increased interest costs which such person is required to pay for financing the acquisition of any such comparable replacement dwelling. Such amount shall be paid only if the dwelling acquired was encumbered by a bona fide mortgage which was a valid lien on such dwelling for not less than one hundred and eighty days prior to the initiation of negotiations for the acquisition of such dwelling. Such amount shall be equal to the excess in the aggregate interest and other debt service costs of that amount of the principal of the mortgage on the replacement dwelling which is equal to the unpaid balance of the mortgage on the acquired dwelling, over the remainder term of the mortgage on the acquired dwelling, reduced to discounted present value. The discount rate shall be the prevailing interest rate paid on savings deposits by commercial banks in the general area in which the replacement dwelling is located.