Rarity depends on how many were made and how many have been destroyed or locked up in collections.

VALUE is determined by rarity, but also by other factors, mainly demand. Age, popularity of the casino, number of collectors interested, location of the casino, denomination, and in some cases, how attractive the chip is (picture chips vs hot-stamped for example) enter into Value. Many older free play or tournament chips are very rare but not particularly valuable because only a few of us go after them. In contrast, Sands last issue dollar chips are far from rare, but have inordinate value because of the popularity of the casino name.

I'd generally agree with James' definition of Rare, which is 75 or less available to the hobby.