On September 11, 2008, about 1630 mountain standard time, a Robinson Helicopter R-22, N227GH, impacted the taxiway and terrain during a practice 180-degree autorotation at the Ernest A. Love Field Airport (PRC), Prescott, Arizona. Guidance Helicopters, Inc., was operating the helicopter under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The certified flight instructor (CFI) and the student pilot undergoing instruction (PUI) sustained minor injuries. The helicopter sustained substantial damage to the rotor system and tail boom. The instructional flight departed PRC about 1545. Visual meteorological conditions prevailed, and no flight plan had been filed. The CFI reported that they had performed four 180-degree autorotations successfully. The final 180-degree autorotation was progressing normally until turning final. At that time, the helicopter experienced a sudden drop in airspeed approximately 150 feet agl. Airspeed indicated 52 knots, and rotor rpm was 101 percent. The PUI responded by pushing forward on the cyclic. The rotor rpm dropped to 95 percent, and the helicopter altitude was down to 75 feet agl. This resulted in lower rpm's and an excessive descent rate. The CFI attempted to perform a power recover, but the rpm's did not build quickly enough to avoid the skids striking the taxiway. The CFI leveled the helicopter before impact. The right front skid contacted slightly first. The rotor then struck the tail boom, and the helicopter entered a right spin. After spinning several times, the helicopter drifted to the left, the skids contacted sod, and the helicopter rolled over. The PUI confirmed that they were performing 180-degree autorotations. As they rolled out on final, the airspeed started to drop. The PUI pushed forward on the cyclic and applied a little down collective. The helicopter descended quicker than the PUI expected, with the rpm dropping below 101 percent. Both the PUI and the CFI rolled on the throttle and leveled the aircraft. The helicopter hit the taxiway with the right skid first. The helicopter started to spin until it hit the dirt and rolled over. The pilots reported that there were no mechanical failures or malfunctions during the flight.