I understand the frustration when you expect an intelligent human rep and get monkey-like (or worse) responses. I have had similar experiences with non-standard inquiries here in Switzerland.
This is just one example of an overly secretive culture that tries to be on the safe side but burns bridges instead.
Any plumber, electrician, etc. can give you a rough cost estimate and they don't even expect business without giving a quote. Sure, there are problems that can arise, but anyone who would instist on not giving any quotes would certainly lose business.
I suppose that by building a service monopoly (not selling spare parts to third party repair shops), Nikon can afford to treat their customers badly, but there must be some negative consequences for them that they don't even realize. It's certainly funny (in a weird way) that Canon can do so easily what seems impossible for Nikon.
They presumably have a lot of statistics about the cost of routine service jobs, like swapping elements or rubber patches, so it is perfectly reasonable to expect them to share that information.
Maybe a rental company could answer the question for you. They have similarly large statistics about repair costs (after all, they need to assess the risks in order to calculate rental fees), and they shouldn't have any incentive to hide that information.