The expense fund shall be the fund to which shall be credited all money provided to pay the administration expenses of the retirement system and from which shall be paid all the expenses necessary in connection with the administration and operation of the system. Annually the board of trustees shall estimate the amount of money necessary to be paid into the expense fund during the ensuing year to provide for the expense of operation of the retirement system. Such estimate shall be provided by the board of trustees from interest and other earnings on assets of the retirement system.