Rarity? Supply and demand? Desirability? I'm thinking it all boils down to how much a person is willing to pay to fill a hole in their collection. For example ... there have been 4 "The Patio" $5 chips sold on Ebay thusfar this year. They are classified as R10 (Rarity) ... but the seller indicates he (she) has had 8 of the chips (hardly and R10 though listed as such). Therefore ... info on rarity can't be used to establish the value of the chip. The chip is also listed as an X chip ($600-699). One sold in that range in January on Ebay, but the last three sold for $300 - $400. For me it boils down to simply a question of how badly do I want the chip for my collection. The "marketplace" varies tooooo much from chip to chip and from month to month. Add that to the lack of true information about "rarity" and it boils down to desirability of the chip to the individual collector. Just my two cents!