Let my start off by making point in advance (so you'll know whether read or skip on). The wizard is by grace of his abilities by far the 'richest' character class of them all, who can spend a lot more money on the stuff he needs then anyone else.
If you just build a character of a given level and spend the fixed amounts a wizard is indeed worse of for having to buy extra spells. But this is for 1 reason only, because a specific xp amount was chosen which happens to correspond to the absolute minimum for the level. The wizard can't spend any xp's on items he could have made himself (depending on which item creation feats he took). If a random xp amount for that level was taken every extra xp can be turned into 'goodies'. (As is the case with 'evolving' chracters which go through the levels).
The counterpoint is always that "the wizard" will lag a level behind. This point is either false or highly overrated (I'll get back to that later). First an illustration of what I mean:
A wizard who spend 10% of gained xp's on item creation is actually converting xp into wealth. He either makes them for himself (cutting cost in half, having more to spend on other things) or for others, who pay him twice what he spends, making him additional money to spend on stuff (so besides his share of the loot). Come to think of it, those scrolls and wands that are so useful for sorcerers to compensate their limited spellbase, guess who they''ll be buying from? That's right, wizards mainly. Extra money for them to buy stuff and inscribe spells

Anyway, this wizard who spends 10% xp, will actually be converting xp into gold piece value at 12,5 gp per xp.
This come down to:
up to lvl 2 100 x 12,5 = 1250 gp
up tp lvl 3 300 x 12,5 = 3750 gp
up to lvl 4 600 x 12,5 = 7500 gp
up to lvl 5 1000 x 12,5 = 12500 gp
....
up to lvl 10 4500 x 12,5 = 56250 gp
....
up to lvl 15 10500 x 12,5 = 131250 gp
A few remarks on this table; the amount is total additional gp at that point; the first two entries are more cosmetic then practical since only 'scribe scroll' and 1 st lvl spells (and cantrips) will be available for item creation. At third level however the brew potion, craft wondrous item feats become available (and no, I'm saying you can get them both at this point) and second lvl spells kick in.
Lets put these amounts next to starting funds for these levels:
lvl 2 900 + 1250
lvl 3 2700 + 3750
lvl 4 5400 + 7500
lvl 5 9000 + 12500
lvl 10 49000 + 56250
lvl 15 200000 + 131250
At lvl 10 you'll have more then double the amount to spend as the sorcerer (by 20th lvl you'll have 'only' 20 % more, but in absolute numbers you'll have over 250000(!) gp more to spend).
Back to the argument of the lagging a level. Remember that this wizard spends 10% of earned xp, so he'll have ten percent less then the rest of the party. But whether you have 6600 xp or 6000 xp you'll still have the same level. It only matters when the level break is between what you have and what the rest has. How often does this happen? About ten percent of the time. So by spending this xp the wizard can vastly (and I mean vastly) increase his wealth. And about 1 in every 10 sessions he'll be a level behind.
Now this is the "highly overrated" part of the rebuttal, here it is assumed that the different chracters will be making the same amount of xp, irrespective of the items etc one has.
We all know that having more goodies greatly increases your changes of defeating baddies and getting xp for it. In case of individualised xp the wizard with all his extra wealth will have it a lot easier, can defeat more and gain more xp. In this case he may actually catch up and pass the others by, despite spending on items, in which case the argument of 'lagging a level' is false.
I know, I know, you'll be wanting to tell me that it takes time and this and that. That's true but all those elements are in the realm of campaign specifics; needing the time to make items is not a problem unless the campaign (DM) makes it so.
The point I made was that a wizard can be the wealthiest character BY THE GRACE OF HIS ABILITIES.
Whether or not he gets to exploit these abilities to the fullest is a campaing specific, as it is for all character classes. A ranger has a favored enemy but the campaign (DM) decides whether he could actually run into them.