(2) Where the consideration, or any part of the consideration, for a conveyance on sale consists of money payable periodically for a definite period exceeding 20 years or in perpetuity, or for any indefinite period not terminable with life, the conveyance is to be charged in respect of that consideration with ad valorem duty on the total amount which will or may, according to the terms of sale, be payable during the period of 20 years next after the day of the date of the instrument.