The actuarial cost represents the current value of the expected improvement in your pension resulting from the buyback of credit. Generally, the closer you are to retirement, the greater the actuarial cost to buy back credit.
Credit for other employment, repayment of refund and repayment of commuted value are all examples of absences that can include an actuarial cost.
The actuarial cost is based on assumptions about interest, inflation and mortality rates, and salary increases. It also takes into account your age, credit and salary.