S.D. Codified Laws § 3-12C-504
A current contributing member of the system may receive credited service by election to make, or have made on the member's behalf, contributions, based on the higher of the member's current compensation, or the member's final compensation calculated as if the member retired on the date of election, at an actuarially- determined percentage times the member rate, for each year of service for which the member wishes to receive credit, if:
The amount of the credited service and the rate of contribution shall be at class A rates unless the service for which credit is sought was rendered as a class B member in which case class B rates shall apply. If a participating unit has failed to pay employer or member contributions to the system on behalf of a member as required under this chapter or under any predecessor system consolidated pursuant to § 3-12C-1601, the amount due the system shall be calculated in accordance with this section.
The member rate in effect as of July 1, 2001, shall be used in calculation of the purchase cost of any service performed prior to July 1, 2002, if a contract to purchase such service is in place prior to July 1, 2004. The member rate in effect on and after July 1, 2002, shall be the basis for calculation of the purchase cost of any service if the contract to purchase such service is not in place until on or after July 1, 2004.
SDCL 3-12C-504