The Bandit is a good example that parts availability has nothing to do with the technology used and everything to do with the market. My bike is actually a 2001 Bandit 600. The 2nd generation 1200 sold well in the United States, the 2nd generation 600 never did. The 1st generation 600 did sell. So there are a ton of aftermarket 1st generation 600 parts availabe in the US, but pretty much the only new replacement parts you can get for the 2nd generation 600 here (that are not in common with the 1200 or SV650, if you can figure out which ones those are) are rebuild kits for the off-the-shelf brakes and carbs, and you can get the gaskets cut. Everything else you either have to import from the UK or get custom-made, which usually ends up being cheaper (things I have had custom made: throttle cables, fork tube).
Do you mean CB550? If so they demand a higher price because they are 1) older and 2) look way better. Super Nighthawk performance isn't much to write home about relative to newer bikes for the same price and vintage bikes look really nice.
Now is probably a great time to pick up a Super Nighthawk because as with almost all vehicles their value drops off continually for their first 20 or so years until they are truly part of a previous generation of vehicles that is no longer available. Then the value starts to rise again due to scarcity.
We saw the same thing in the 90s with 1970s domestic cars which peaked in the 2000s. It's happening now with 1980s Japanese cars and has been happening to 1970s and 80s Japanese bikes.