Li Shenron
Legend
I found a chart a while back that gives you the "average" wealth by level chart. Really wish I could give credit where credit is due, but I don't remember where I found it. As a bonus it also gives average treasure per level.
This is based on using standard encounters, average treasure results, etc and is by no means an official guideline and I only use it as a general rule of thumb.
I think this is the only way to answer the original question: given that the game doesn't explicitly specify wealth-per-level values, just take all random treasure tables and figure out the average treasure value accrued by the number of (average) encounters expected before each level. Since there is both a range of possible encounter difficulties and a fairly wide range in treasure results themselves, it's likely that you'll end up with a large variance on the final result, but nevertheless you'll have an average to work with.
However, that would tell the total earned treasure, not the total owned, to figure out which you might have to deduct the expenses during previous levels, and here things get more murky... how much gp are you going to assume that the characters have spent on healing potions and other consumable items? How about expensive spells components? How about money spent on living (a rule that is not even so commonly applied but rather ignored), or invested in other ways during the adventures (mercenaries, favors, bribes, building, transportation, services, information...)? Everyone can have their own expectations for those, but really they are all pretty much made up numbers, and for some gaming groups they may even be 0.
And of course finally what really matters is how the DM is going to allow the PCs to spend the treasure. Thus, I wouldn't really recommend to sweat much over these calculations/estimations. Even just granting as many GP as XP yields usable values!