I love the fact that you guys enjoy digging deep into the economics.
By having a spellbook, you avoid the cost of having to research the spell yourself. So the value of a spellbook is equal to the cost of learning a spell without a spellbook. What is that cost?
a) A scroll with a market cost of 1,000gp/level will suffice to learn a spell.
b) Conducting spell research costs 1,000gp/level plus 500gp/level in special components plus the mage’s time (~~ 2,000gp/week for an assumed 9th level mage). So for spells not available on scrolls, the cost would be 1,500gp/level (base + special components) + 4,000gp/level (mage’s time, where each spell level takes 2 weeks) or 5,500gp per level.
I will presume that spellbooks cannot be rented due to (a) the risk of theft and casualty, and (b) the desire of spellcasters-for-hire and mages guilds to avoid having magical secrets spread too widely.
Since you can’t rent it out, once you’ve used the spellbook, it’s of no further value to you. So the value of a spellbook is equal to 1,000gp/level for common spells and 5,500gp/level for rare spells. [To the extent you think spellbooks would be rented out in libraries or ‘spellbooks-as-service’ the value might be more, or less, depending on supply v. demand.]
Thus per the above example, lets assume an 8th level mage with full spell repertoire and no INT-bonus: 3x 1st level spells, 3x 2nd level spells, 2x 3rd level spells and 2x 4th level spells. All are common spells.
3 x 1000 x 1 = 3000gp
3 x 1000 x 2 = 6000gp
2 x 1000 x 3 = 6000gp
2 x 1000 x 4 = 8000gp
This is certainly a nice sum, but its not a king’s ransom. It’s about 4 talents of gold, or less than the cost of a tower.
In contrast, let’s consider the dreaded iron-bound Books of Sebek, which holds the terrible and secret spells that evil sorcerer-king created in his life. Let’s assume Sebek is a 14th level caster, that his standard repertoire had common spells, his INT bonus was used on custom spells. The Books of Sebek would be worth:
4 x 1000 x 1 = 4,000gp
4 x 1000 x 2 = 8,000gp
4 x 1000 x 3 = 12,000gp
4 x 1000 x 4 = 16,000gp
3 x 1000 x 5 = 15,000gp
3 x 1000 x 6 = 18,000gp
3 x 5500 x 1 = 16,500gp
3 x 5500 x 2 = 33,000gp
3 x 5500 x 3 = 49,500gp
3 x 5500 x 4 = 66,000gp
3 x 5500 x 5 = 82,500gp
3 x 5500 x 6 = 99,000gp
A vast sum, no doubt, but there will be perhaps 1 Sebek-level mage in each empire.