First of all, helicopters are more expensive to buy than airplanes of similar size and capacity.  They are also more complex and consequently more expensive to insure and maintain; there are more moving parts (like main and tailrotor gearboxes) to service.  On a typical small airplane, the engine and propeller are the only components that require a periodic overhaul after a certain number of flight hours.  On a typical helicopter, however, most major components have a "timed life," or time-between-overhaul (TBO).  Some components have to be replaced with new components after so many hours of service.  For example, the main rotor blades on a helicopter have to be replaced every couple thousands of hours of flight time.  Each new blade can cost tens of thousands of dollars.  These extra costs have to be factored into the total hourly operating cost of the helicopter.  It will typically cost about twice as much to fly a small helicopter per hour than a small airplane.