The first time this came up was when my ad-hoc dungeon-crawl-type universe (Dreamspace*) transitioned to Fate 3 (from D&D 3.0 with a brief stop at Burning Wheel). Adventurers generally don't have any regular income or resources to fall back on; they generally loot treasure from the necromancer's hideout, then go to town and spend it all on magic items for their next adventure. What I ended up doing in the Dreamspace campaign was giving people "dots" of treasure. Dots of treasure add to your resources, and when you fail a resources roll, you can give up a dot of treasure to get the item anyways. This still adds randomness, but in a universe where magic items are mostly unique and don't necessarily have D&D-style consistent relative difficulty to create and market value, it worked OK, especially without worrying about resources for things like sub sandwiches.**
Now, starting a Dresden Files RPG campaign, Sam and I ran into a similar issue. According to the rules, a person with Mediocre resoruces (on average only able to afford nice clothes or a tent) who decides they want a compact car has more than a 1% chance of being able to afford it, and after buying that car their ability to buy future things is not impaired. Obviously it's possible to use common sense and lucky coincidences to make the results of resources rolls make sense if you try hard enough, but it doesn't really reflect how buying things works in the setting (superficially modern America). If I (note: not penniless) wanted to buy a compact car, there'd be some randomness in the process, but it wouldn't be about whether I could afford the car: it'd be about how nice my car ends up being and how much the loan I'd have to take out would constrain my future finances.
So, the way it makes sense to represent this in Fate is based on a mechanic from Diaspora, a hard SF Fate RPG that's probably too hard genre-wise for me to ever end up running it. In Diaspora, you have a Wealth stress track, the same way you have a Physical stress track that you take hits on when fighting and a Social stress track that you take hits on in social conflicts. Diaspora still has a Resources skill (called Assets), but if you try to buy something, you always succeed: if you failed your roll, you simply take stress, reflecting the impact on your financial situation. Like with other stress tracks, if your stress track fills up you start taking consequences representing your financial problems. This makes a fair amount of sense, and deals with the independence of Resources rolls in a relatively simple way.
I still feel like this has too much randomness for average purchases in DFRPG. The current mechanic I have floating in my head is as follows: your Resources modifies a Financial stress track a la Diaspora. When you want to buy something that's widely available for standard prices (anything you could get in a chain store), you don't roll Resources at all. If your resources is high enough to match the item's price, you can buy one easily without any impact on your finances.*** If it's more expensive than your resources, either don't buy it or take Financial stress equal to the difference. You can't buy it if you can't check off the stress box. You can use consequences to enable you to afford things that you otherwise couldn't, reflecting things like being buried in loans or pawning something important or whatever makes sense.
Stuff that's not easily available for standardized prices, like drugs, black market goods, or ancient artifacts, you might still roll resources for. The dice roll would ideally be saved for the adventure to keep prices of the same thing from varying too much.
This stress doesn't go away quickly, like stress on normal tracks; my initial take on recovery is that each significant milestone (i.e., end of adventure) you can either erase all boxes equal to or below your resources, or one box higher than that. The other option, of course, would be to get someone with sufficient Resources to pay you enough money, which would have to be for some non-standard job that could kick off an adventure Harry Dresden-style.
I feel like this does a better job of representing both how money works in real life and how Harry's constant debt feels in the books, without getting bogged down by counting change like D&D does. I don't actually know if I'll actually use this mechanic in Thin Ice, but I'll certainly think about it.
And if any of you Boston-area fellow mongeese are interested in a Dresden Files campaign, there's still time to get in on the ground floor. Minor practitioners and academic rivalries and fae, oh my!
*: Shared with the fiction blog I lost direction on extremely quickly.
**: Sub sandwiches are not currently canon for Dreamspace, but they'd fit right in with the yoyos.
***: I'd use a quantity ladder, along the lines of the time increment ladder, if you want to buy several of something; one tent would cost 0, a few would cost 1, ten would cost 2.