Hi, I noticed that the tomes that grants bonuses to stats (understanding, leadership  & influence, etc) have less than double the cost of buying a tome than the actually creation cost. Isnt the standard rule that double the cost of creating is the cost of purchase?
EG: Tome of Understanding Cost 26,250 gp (+1), 52,500 gp (+2), 78,750 gp (+3), 105,000 gp (+4), 131,250 gp (+5) to create but its Price is 27,500 gp (+1), 55,000 gp (+2), 82,500 gp (+3), 110,000 gp (+4), 137,500 gp (+5).
As PRD.
Is this correct or an error?