(1) Subject to the requirements of each retirement system, a member with five or more years of membership service may purchase additional service credit. Members may purchase one full year of additional service for each five full years of membership service credited in the retirement system. A member eligible to purchase additional service may purchase full months of service totaling 11 months or less.

(2) The cost will equal the actuarial rate for the respective system times the member's compensation for the immediately preceding 12 months. Each full month of additional service will cost 1/12 the cost of a full year.

(3) The board will include a member's additional service when calculating the amount of a benefit, but not for initial retirement eligibility or for early retirement, except in the following cases:

(a) PERS, which requires additional service to be included when calculating the early retirement reduction; and

(b) the sheriffs' retirement system, which requires additional service to be credited for the purpose of meeting retirement eligibility.

(4) A retired member who returns to active PERS membership may purchase additional service after at least 12 months of active service. The amount of additional service which may be purchased will be based on the member's total membership service in PERS.