Of the two main types of retirement plans, defined contribution plans (like 401k plans) are the easiest to value, as participants are generally provided with quarterly statements that reflect the exact value of a participant’s plan assets at a specific time (although there are vesting issues which may need to be considered).
Defined benefit plans (such as pension plans) are more complicated to value as payout terms and conditions vary, as the value of a pension plan will increase (often significantly) the more years that an employee works for a company and the higher the salary that is paid to the employee.