I was statting up a wizard NPC recently, and I wanted to give him another top tier spell known in his spellbook, but I'm a bit confused about how this affects his wealth-by-level.
If you're designing an encounter with a wizard, subtract the value of a spellbook and material components (see Selling a Spellbook on page 179 of the Player's Handbook) from the average treasure value before you start rolling up treasure. Alternatively, you can add up the value of all the components and the spellbook and compare the total value to Table 3-3: Treasure Values per Encounter. Find the level that most closely approaches that total, and subtract it from the level of the encounter. Use that new level to generate the rest of the treasure.
Okay, what this all boils down to is that the cost of the spellbook and components is taken out of the NPC wealth-by-level total. But how much does it cost the NPC wizard to have an extra spell in his book? I usually just allot my NPC wizards with only the spells they would have gained through leveling up, just to avoid this very question!
Does the wizard have to pay for the scroll he presumably got the spell from, and the cost to scribe it into his spellbook? Does the wizard only have to pay the cost to scribe it into his book? He has to pay something, or NPC wizards can effectively have as large a spellbook as the DM feels like for free, which doesn't make sense. I'm thinking since the wealth-by-level guidelines are intended to monitor the amount of gear NPCs have for the purposes of determining CR and how much loot PCs get off them, the rule doesn't care about where the wizard got his spell. Thus, he wouldn't pay for the assumed scroll he copied the spell from, but rather only the 100gp/page to have the spell in his spellbook. Correct?
Just pay the 100 gp per level, as that's what the DMG says.