"Rare" and "desirable" are different things (and somewhat conflicting.) Let me explain the conflicting part: If something is truly rare (unless it has been cataloged, like a coin or something) not enough people would know about it to become "desirable" to a critical mass of people to drive its price upwards. And with "rare" I am using the definition of "few copies made compared to others" and not what the word has become to mean these days. Paramounts are a nice example of a desirable but not rare bike that commands high prices. 50s-70s Corvettes are a similar example as far as cars go. Desirability has another factor: Something that was perceived to be "high end" and "unaffordable" to the average person (see: Paramount and Corvette.) Back to the car talk: There are some really quasi rare cars, like a 1987 Buick GNX (547 produced), or a 1968 GT500KR Convertible (517 produced). These are a bit esoteric and do demand a higher premium that "desirable" vehicles of the same era (think, Buick Grand National and '67 GT 500 fastback). And then we are talking about the rare cars, like the single 1968 GT500 KR Hertz Convertible produced. Not many people know about it, so it is not desirable, but enough people know about it, to be insured for $4M (that's about 2 zeros more than its rare but more common peers bring in the market.).

Kind of complicated equation, but as far as rarity goes, it is more like this: If you've seen one, it's probably not rare, if you seen 2, it absolutely is not rare, but in both cases, it might be hard to find and desirable (or not).