During the 180-degree autorotation the student maintained 65 knots with the rotor revolutions per minute (RPM) in the green. As the student rolled out of the turn at about 300 feet above ground level and at 70 knots, the rotor rpm continued to remain in the green. However, prior to the flare the rotor RPM began to decay and failed to build as the flare was entered. The flight instructor then took control of the helicopter and immediately rolled on throttle to recover engine [power] and to maintain [rotor] RPM, but there was insufficient engine power to recover the decayed rotor RPM; this was followed by a low rotor RPM light and horn. As power was not available for a full power on landing, the flight instructor initiated a full down autorotation to a taxiway. During the landing the helicopter skidded across and off the wet taxiway into loose gravel and mud before the left skid dug into the ground. This resulted in the aircraft rolling over and coming to rest on its left side. The helicopter sustained substantial damage to its welded frames, firewall, and tail cone. The flight instructor reported no mechanical malfunctions or failures with the helicopter or flight controls that would have precluded normal operation.