It's a complicated equation. Is value determined by price, or is it the other way around? For someone to pay money for something, they have to understand it; so the fewer people that understand something, the fewer potential buyers-- and the lower the price.

Among collectors of American coins, one of the 'holy grails' is the Confederate half dollar. No one knows how many were made, but last I heard four were known to have survived. That's enough for them to be in the coin books, so just about every collector knows they exist, but hardly anyone has actually seen one. The result is they're quite valuable. On the other hand, unique coins -- of which only one example is known-- are comparatively common, and not necessarily worth much.