What you were charged is actually very reasonable imo. Assuming the units were properly serviced by a trained and attentive tech. To properly inspect, disassemble, clean, inspect again, reassemble with proper torque specs, and tune takes time! And thats just the beginning, a good service should always include a few purge cycles or a few minutes on a quick set followed by a leak test and a final check to insure the unit has held its tune. You are paying for a very important service. It blows my mind that people have no problem paying a car dealer $100 per hour for a brake job yet can't seem to understand a few hundred dollars every couple years to maintain breathing equipment.