No guidance is given
Filled spellbooks do not have a defined value but, contrary to the linked Paizo forum post, they are intended to be sold. The GM should determine the value of a spellbook when (before) it is made available to the PC's, such as during encounter building. There are a few tools to reference for that.
- Base value is 1g, plus:
- The Formula table
- [0.5, 2, 5, 13, 25, 50, 100, 225, 500, 1200, 3500]
- The Learn A Spell table
- [2, 2, 6, 16, 36, 70, 140, 300, 650, 1500, 7000]
- The Scroll item
- [4, 12, 30, 70, 150, 300, 600, 1300, 3000, 8000]
Above are the 3 general values of each (common) spell in a spellbook, listed from least to most. I would think no spell should be less valuable than a formula of double its level, but should definitely be less than its value as a consumable scroll. Learn a Spell's table strikes a happy medium and is what my groups use to determine value. This value can then (optionally) be decreased for exceptionally common spells or increased for Uncommon and Rare spells.
The base value might then be halved, as normal, although spellbooks might be interesting enough Treasure to be 'affected by supply and demand' or haggled over.