It is rare today to find systems rated below 13 SEER in the U.S. New government guidelines require a minimum SEER rating of 13 when replacing older devices. SEER ratings can go as high as 21 SEER. The higher the SEER rating, the more efficient the system. The efficiency of your system is increased by 3% - 5% for every SEER rating point.
Of course, the cost for replacement increases incrementally as the SEER rating goes up.