After practicing a series of autorotations, the flight instructor and his student attempted to complete a 180-degree autorotation. The student began a stabilized autorotation from the downwind leg of the traffic pattern. According to the flight instructor, during the turn the student allowed the nose of the helicopter to drop; he was instructed to correct this attitude by applying aft cyclic. When the student completed the turn at an altitude of 200 to 250 ft agl and at a speed of 80 to 85 knots, the flight instructor believed that the outcome of the maneuver was in doubt. The flight instructor then took control of the helicopter and began to initiate a recovery by raising the nose and attempting to increase the throttle and collective to arrest the descent rate. According to the flight instructor, the RPM decayed and the low rotor rpm horn came on at 150 to 100 ft agl. The helicopter landed hard and subsequently slid off the north side of the taxiway. The forward section of the left skid contacted the soft dirt, which resulted in the helicopter turning to the left and nosing over. Examination of the aircraft by a Federal Aviation Administration inspector revealed substantial damage to the helicopter consisted of the separation of the tail boom assembly, damage to the firewall, the left forward frame, and left skid support structure. The flight instructor reported no mechanical anomalies with the helicopter or flight control systems.