I assume they base it on any data they can get about how many are manufactured as well as Ebay sites. I know that one of the NES rarity guides was discussed on one forum and they said the creator(s) checked Ebay for an entire year to see how rare each game was. I know that one forum discussed doing research of their own and each member would choose a set of games to check on every week for a whole year.

BTW, there are 205 Super Mario World carts on sale right now