Michiel said:
Value plays no role. My rarest units were very cheap, just because no one recognised or noticed them. Sorry, but black BB40's and that kind of stuff is hardly rare. Internet is scattered with pictures of them. Also see them a couple of times per year on eBay. They are desirable and mega expensive, but not rare IMO.
I tend to agree with Michiel.
I'd equate rare with models that never sold well or were limited to a particular area of the world, or even short production runs. Many models that had bright coloured cabinetry (ie the red JVC pc-w300 or red Sony D5 cd player) to me are rare because they were sold in much lower numbers compared to their mainstream brothers.
I guess the silver 550, and the gold Conion cold also be lumped into this as well. It just so happens that these models to tend to command a high price because of their rarity, but cost is not the benchmark by which rarity should be equated with.
To me a minty fresh, working SK-95
in my hands is a rarity...because I can't find one
Just my 2 cents..........