Ultimately, what something is worth comes down to demand, coupled with scarcity. Some vintage bikes are valuable, because they were coveted by (many) folks in their youth, that now have the means to buy them. They find there is a finite, and small supply, so they have to pay accordingly. Once those folks pass on, the prices will almost certainly fall. Other things can be extremely rare, yet worth almost nothing. Early computers from the 60s? Room sized, and cost millions (back then). There are probably only a handful left, and they're probably not worth more than scrap prices.