Members who terminate membership with three years or more of creditable service and later return to membership may be given credit toward retirement for prior creditable service, subject to the condition that such member deposit in the pension fund a sum equal to the member’s portion of the actuarial cost to restore such service. The member’s portion of the actuarial cost is determined on the ratio of the member’s contribution rate to the total of the member and employer contribution rates at the time the member elects to purchase the creditable service.