...something with 3rd level spells, it starts to rise in price exponentially, so we will say 2500 GP.
For 4th level spells, 25,000 GP
For 5th Level Spells 125, 000 GP.
6th Level spells...600K GP
7th Level Spells...well 1 Million GP.
8th Level Spells....5 Million GP
9th Level Spells...you better be a good wizard cause they are going to hunt you down and try to kill you for the book if they know it's available...a nice wizard may give you something very valuable in trade...etc.
(could also apply to those books with 7th and 8th level spells as well).