Technically, it isn't what a card sells for. It's kind of tit-for-tat but here is what PSA says about the weight of a card in the set:
Within a set, not all cards and tickets have the same value. Sets in the PSA Set Registry are weighted on a 1 to 10 scale with 10 being the rarest item and 1 being the most common. While it may be in true in some instances that an item may be many times over 10 as rare as the most common item in the set, the Registry keeps the scale simple so that all levels of collectors can compete. Each item within the set is assigned a weight based on the value of the item in NM-MT condition.
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