The expense fund shall be the fund to which shall be credited all money to pay the administration expenses of the system, and from which shall be paid all the expenses necessary in connection with the administration and operation of the system. Biennially, the board of trustees shall estimate the amount of money necessary to be paid into the expense fund during the ensuing biennium to provide for the expense of operation of the system, and shall pay that amount into the expense account from the investment earnings of the system, subject to review by the legislature and approval by the governor.