At approximately 1630, a Cessna 650 suffered failure of its right wing aileron cable while in cruise flight at FL390 approximately 200 miles southeast of Rapid City. The airplane landed without further incident. The pilot reported the airplane was in cruise flight with the autopilot engaged when it suddenly rolled to the right. He disconnected the autopilot and rolled wings level, noting that it took left control wheel deflection to maintain wings level. Initial inspection revealed the right aileron cable failed near the outboard pulley just prior to the bellcrank.