If the report of the board of directors shows that the proposed project or any unit thereof, is estimated to cost more than one million dollars ($1,000,000), the board of directors may provide that any bonds issued to finance the same shall be extended over a period of not to exceed 15 annual installments, in which event the initial payment from each county within the district shall be not less than the sum obtained by dividing the total amount payable in each county by the number of installments in which the bonds are to be paid.