The rarity is calculated by pressing amount and how often the record appears on the market. The value have no influence.
1/5: Very Common (pressed in many quantities / often seen)
2/5: Common (pressed in quite many quantities / appears from time to time)
3/5: Rare (~1000 copies / hardly seen)
4/5: Very Rare (~500 copies / rarely seen)
5/5: Extremly Rare (1-400 copies / never or extremely rarely seen!)