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I have a couple of 500's, never owned an R22 but loads of H300's all I can say I would be gob smacked if H500D was cheaper but having said that it depends upon how you compare.
EG fuel in Uk avgas is about £ 1.50 a litre and jet about .55p per litre
therefore R22 uses about £ 52.50 worth of fuel an hour MD 500 uses £ 62 per hour. But if you now ask about fuel cost per mile that changes things
In 1 hour R22 covers about 80nm 500 about 130 nm. Therefore for R22 to cover 130 nm fuel cost is £ 85 so you can argue that the turbine is cheaper on fuel !!!
Obviously comparing hourly costs is easy but per seat mile may be better as the MD wins due to its greater speed so you use less fuel less component times etc etc. Plus you have no time life components unlike Robinson or EC Airbus products
Basically you can make figures say anything you want to !!!