It is a bit of a strange rule situation.. but it does lead to the question: When you have a higher level wizard do you count spells in their spellbook against their accumulated wealth? If so, is it literally possible for a character to have no other items aside from his spellbook? Does anyone think that is actually balanced?
I look at it as a temporary cost that happens along with play.. the bigger part of the cost is 'time', but that is hard to measure when starting above a certain level.
Counting spells placed in a spellbook against total wealth seems to me to be much like counting the cure light wounds potion that they drank 8 levels ago against their wealth, although not quite as bad.
For my own games I generally just assume that they have placed a few spells in here and there along the road and some of the wealth might still be missing.
As such, I let them place in double the normal amount that they would have for the level for 'free' and then pay half scroll cost + half writing cost for any additional spells that they want.
It still limits the character to how many they can have, but it doesnt unduly punish them and it allows the wizard to actually thrive in the area that he is supposed to thrive.. number of spell choices.
It is a houserule of course, but I think it fits the bill much better than the normal 'doing it right now' effects.. After all, why would it hurt your wealth 'now' to have placed that spell in your spellbook 10 levels ago? (especially when 10 levels ago might be 20 years ago in game time)