Short for "family prices". The term is used by hippies and music festival attendees to assure potential customers (custies) that the products being sold (shirts, jewelry, drugs, etc.) are at fair, almost family-like, prices.
However, majority of the hippies using this term sell their products over-priced and often times unreasonably high. It is a deceptive term to those unfamiliar with the Festival scene, making individuals feel as though they're getting a good deal when in reality they're getting ripped-off.
The term is very ironic, being that it is supposed to signify a person's family-like actions when selling a product; the truth of the matter is that the seller is tricking the customer and portraying very non-family-like behavior.