WBL Covers This
'Wealth By Level' is not 'how much treasure you should hand out'.
It's how much wealth the players should have on them. So if they spend 75% of their loot on parties, you need to drop 4x the treasure somehow.
Problems With This;
It's pretty metagamey.
If one PC throws huge parties and another doesn't, and they normally split treasure equally, you're in a pickle (Solution: Sunder the rich guy's gear, or find something for him to spend that extra money on).
Keeping track of people's wealth is a pain in the a.
Positive Consequences of This;
The Wish Economy
There is a concept, created by Frank Trollman and K which is called the 'Wish Economy'.
It posits that after a certain level of power, it's too easy for the Enchanter to mass-mind-control the treasuries of small nations, or the Necromancer to animate a giant workforce to plumb the poisonous depths of the gold mine, or the Conjurer to call Earth Elementals and Bind them to mine the Plane of Earth for a year and a day for precious metals en-masse, so forth and so on. If creative players can get more gold than they are supposed to, and directly convert that gold into power, it can be awkward for the GM.
Conversely, if the GM strictly limits how much gold the players have, then any skerrick of money the player spends on anything other than adventuring gear is 'wasted'. Either you're metagame managing their money (as the WBL system wants you to) via treasure drops, or you're letting them become king-gods of the magical item system, muhahahaha, haha, ha.
The Wish Economy's answer to this is to essentially split the DnD economy into two halves. The 'Gold Economy' which contains Minor magic items and potions and things, which can be bought for gold, and the 'Wish' Economy, where things like wish 'vouchers', souls, rare planar substances (the stuff used to create magic items) and Medium and Major magical items are bartered in mafia-style backroom deals in planar metropoli. It's not a hard and fast rule - it's fine for the occasional Major magic item to be sold for gold, by some hick Material Plane rube who doesn't know any better, but the idea is that after a certain level (8-11), players stop caring about gold as much since it can't buy the things they want to buy. So they can use it to build fortresses and hire priests to minister to their lands and hire armsmen and pay for wild parties etc, because the gold is not worth as much to them at that point. Or they can buy huge wads of minor magical items, or scribe heaps of scrolls, or whatever, if that's their bag - but either way, it doesn't change their overall power that much, since at level 11 a Minor Magical Item is small beans.
The Wish Economy assumes that 'free' wishes from Genies and things are being directly converted into wealth which is why the bottom falls out of the gold economy at level 10, but there are many ways for spellcasters and rogues and even Fighters to make inappropriate levels of wealth once they get into the middle levels, so it really covers both scenarios.
- Assumes spellcasters etc are trying to leverage their abilities to create wealth and power in the setting. There is an unspoken rule against this in some games, to create a medieval pastiche setting, so it won't suit settings or games like that.
- Assumes, to some degree, a planar multiverse, or a setting with a large enough number of high CR people and Major Magical Items to support an economy based on that.
- Decouples 'Gold' and 'Power' at high levels, thankfully, finally.
- Creates interesting plot hooks by creating a 'planar economy'.
- Allows you to have things like a 'dragon hoard' that is actually the size of Smaug's in The Hobbit without making your players gods.
- Gives the party an excuse to build a giant fortress and army even though it's suboptimal normally.
- Lets you tell stories about leveraging economic power without the PCs just taking it all and dumping it into a magic mart to get personal power.
- Is pretty fun to roleplay, since getting a major magic item explicitly becomes a planar shopping trip with associated mini-quest every time.