A spellbook fully filled with spells costs ~10,000 gp (100 gp times 100 pages minus free spells at first level and two new ones per level) and is only worth 5,000 gp on the open market, but it seems some DMs want to charge a per scroll price for each extra spell contained within. This is okay if the spells are only 0 or 1st, but quickly gets out of control for higher level spells because scroll cost involves spell level times caster level. Do these DMs count used potions as part of character wealth three years later? What if the character still has the empty potion bottle? It is actually worth something (not what it cost but it does have worth). I think tracking such things as original worth for items that are no longer worth that price is unreasonable and stingy.
If a fighter's sword breaks; does that still count against his character wealth? It is not worthless; it can still be re-forged and re-cast as a new weapon without having to pay for the materials. Its worth has severely dropped in value, but so has a used scroll.
A wizard can also copy from another's spellbook at the suggested measly price of 50gp/spell level (which is the actual worth on the market).
Making copies of spellbooks also gain a time and price reduction so that it only takes 1/2 day per spell and 50 gp/spell level for the copy. Stupidly, these copies sell for their full value as no one can tell the difference from a copy (1/2 cost) or an original (full cost).
If counting the spellbook as character worth, the actual worth of the spellbook on the open market should be used: 50gp/spell level, max 5,000 gp per spellbook (plus protections).
I have played with a DM who insisted that the cost of used scrolls counted for total character wealth even if found on an adventure. Potions and wands did not get the same treatment and neither did broken equipment. To be a wizard I needed to be a potion-drinking, wand-using, nearly-non-spellcaster. He also insisted that a spell component pouch came empty and had to be filled "in game"; I needed to keep up with how many castings I had of each spell that required a material component. When no price was available for a material component he required a minimum of 10gp/spell level/casting. I found another game.
Ciao
Dave