The stated value of spellbooks in the PHB is 1/2 the cost of scribing the spells into the spellbook.
From the SRD:
Selling a Spellbook
Captured spellbooks can be sold for a gp amount equal to one-half the cost of purchasing and inscribing the spells within (that is, one-half of 100 gp per page of spells). A spellbook entirely filled with spells (that is, with one hundred pages of spells inscribed in it) is worth 5,000 gp.
Yes and wouldnt you agree this follows the same patern that say a +1 sword is worth 2,000 and you sell it for 1,000. Again, the spell book is worth 10,000 and you can sell it for 5,000 as usual for selling items in the d&d world of mercantilism. It is not 5,000 sell it for 2,500.
I think a per page thing is the best approach for spell book value. Giving different value for each spell in the book- is not right- much too complex.
I am going to use this:
Each spell takes up 1 plus the spell level in pages.
It will cost 10gp/page to scribe a spell.
The overall spellbook will be valued at 10gp/page. (A full spellbook will have a 1,000gp value and sell for 500gp. ) (Also in essence, 0 level spells are valued at 10gp, 1st level at 20 etc. all the way down to 9th which are valued at 100gp.)
Those values seem a little cheap. Thanee, You like 50gp per page?
At 50gp per page a full spell book would be 5,000gp --> that seems doable.
So lets see a first level wizard would have (as the book) 3 + int 1st level spells and all cantrips.
Under 1+ spell level for each page would be --> 28 pages for 0 level and 10 pages for first would be 38 pages or 1,700gp spellbook. Zykes! That still seems a bit high.
I think the problem is that low level spells seem to be worth too much and high level spells seem to be worth not enough.
How about changing the page count to be double the spell level or 1. So an 9th level spell eats up 18 pages and 0 eats 1 and a 1st eats 2 a 3rd eats 6 etc.
Then change the per page value down to 25gp or even 20gp (for simple math reasons). So a 38 page spellbook would be worth 760gp. That seems a lot more like it.
I agree with that, and even moreso, the whole scribing rules are totally whacky.
On the bright side, at least it is better than second edition. spell level+1d6 for each of your spells. Let me tell you it took a lot of time to manage your myriad of spell books!