Rarity is often associated with value, but that isn't always the case.
While a 1969 Mustang coupe with a low-compression 302 engine and cruise control is quite rare, it isn't sought after. On the other hand, well over 10,000 Shelbys were produced, but they command very high prices in the marketplace.
There are cases where a rare option enhances the value of a vehicle. Take 1968 Shelbys, for example. Recently, an article by me was published in Mustang Magazine. It detailed some of the rare options that customers managed to get installed, even though these options weren't supposed to be available. See here
for a copy of the article.