It’s not so much the parts side, but the operational side of a helicopter. I think a better term would be “repair/refurb” as a majority of helicopters are still working for a living rather than someone’s “weekender” to pass the time. Heck, the last military UH-1H Huey just left active-duty status in Dec 2016.
So it’s a different demographic than an old Stinson or Stearman as most helicopter owners maintain their machines at a higher level and fly more than 100 hours a year. In comparison, a current popular helicopter model with private owners (Robbie R22) has a shelf life of 2200hrs/12yr before it needs a complete factory rebuild. So there is no option not to "restore" it. But this is the exception not the norm.
There are some models that have been actually “restored” like older Bell 47s and the original Sikorsky R-4s (UK Hoverfly above) but mostly by museums, associations, etc due to the cost of such work and no ROI. But there are some nostalgic private owners out there who have spent the coin to do this. On the other hand, someone bought the TC for the 47 a few years ago and has started building new Bell 47s. So there may be a resurgence on that model.
Throw in the amount of rotable and life-limited parts on a helicopter and you not only increase the expense but reduce their availability. The first things to go on legacy acft are transmission cases as none have been produced in eons. Then there is traceability of parts especially if a particular model had an equivalent military model like the Bell 205 vs the UH-1. The whole bogus part/salvage rebuild issue was dealt its final blow by several asinine helicopter shops looking to make a quick buck.
So yes its more expensive on one side, but it's harder to spend that kind of money just to fly it 100hrs a year or sit parked when some things time out calendar wise, as compared to say rebuilding a vintage Stearman where your $75,000 ground up restoration is good for the next 50 years if you take care of it.
