As I mentioned in my last post, I've got a vague but persistent world for my Dungeon Fantasy game. It's vaguely defined but it's a sandbox within the PC's immediate area. I figure it's broad enough that I can keep setting any future DF games within it, regardless of the addition or subtraction of special powers, races, technology, and other stuff.
I like the idea of players affecting the setting. Their home base for the local megadungeon/big damn dungeon is the 20,000 person city of Stericksburg. I decided the make the place easily affected by the PCs. So I'm currently tracking the city of Stericksburg and the spending of the PCs.
The idea is that PCs spending money in town increases the value of the town. It doesn't matter how much you loot or hoard, it matters how much you spend in town. For example, right now I set a few die rolls I can use when I want to know if a given odd item or adventuring-only is available.
No roll: Basic items.
15 or less: slightly odd items.
12 or less: adventuring items.
9 or less: special order items (spider silk rope, crossbow sights, potion belts, etc.)
6 or less: lesser magic items
Unavailable: special magic items
Anything people want to buy, I roll against.
I've got some (currently vague) thresholds for improving those numbers. Once the PCs spend enough money on upkeep, bribes, carousing, etc., the categories shift up one each. So the more the PCs spend, the better. You want the city to support multiple alchemists? Keep spending money and sacking the dungeon and more guys will brew more elixirs and more of them will be kept available for sale. This encourages PCs to squander their cash, order crazy stuff, and - yes - upgrade their gear. The "wasted" money might count for more, to encourage people to blow cash on the intangible partying over new swords, etc. But any way the spend it, it increases their access to gear they want and need.
This will be similar for services, hireling availability, access to the upper class (everyone loves a big spender), etc. The more they spend, the better the city gets.
I know I should factor in players buying, but I figure, once money starts flowing in, everyone will produce more in the hopes of getting a piece. Plus it's simpler. The players know this, so they have a direct incentive to spend, spend, spend.