S.D. Codified Laws § 3-12C-509
If a current contributing member with at least five years of contributory service in the system has other public service for which the member is not entitled to retirement benefits from another public retirement system, the member may elect to deposit or have deposited on the member's behalf an amount equal to an actuarially-determined percentage times the Class A rate of contribution multiplied by the higher of the member's annual compensation at the time of making the election, or the member's final compensation calculated as if the member retired on the date of the member's election, for each year of other public service for which the member wishes to receive credit as a Class A member.
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 was 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-509