The board and the local agency may agree upon the terms and conditions of payment of rentals, which shall be fixed at an amount which will return to the State within the period agreed by the board and the local agency the sums which the State, through the board, has expended for the local agency, plus an amount to be specified by the board to be in lieu of the interest which the State would receive if the money were to be invested by the State. The maximum term of an agreement shall be 20 years. The board may require a shorter repayment period.