At least ninety days before each regular session of the general assembly, the board shall certify to each department an actuarially determined estimate of the respective shares of each employer in the amount which will be necessary during the next appropriation period to pay all liabilities which shall exist or accrue under sections 104.010 to 104.270 during such period. The estimate shall be computed upon a level percentage of payroll compensation to cover the normal cost. To the estimate actuarially computed there shall be added the additional payment of ninety dollars per month due those retired members of the highway patrol who have not attained the age of sixty-five years. Each department shall include in its budget and in its request for appropriations for personal service, the sum so certified to it by the board, and shall present the same to the general assembly for allowance. The sums so certified and appropriated, when available, shall be paid to the system and deposited in the transportation department employees’ and highway patrol retirement and benefit fund. Such contributions and contributions previously made by members are the funds of the system and shall not be commingled with any funds in the state treasury.