Idle pulley $36 BMW or $25 OEM, complete AC tensioner $56 OEM or $90 BMW, complete water pump/alt mechanical tensioner $53 OEM. Some have updated to the hydraulic tensioner which was a BMW update, runs $100 for OEM. If you just want to replace the pulley on the mechanical tensioner(s), get Dayco 89133 from Amazon for $19 or Autozone sells it under their brand name to. Made by OEM INA. BMW prices from discounting BMW sponsor TheBMWPartStore.com and OEM from Autohausaz.com.