Some thinking-out-loud on the subject of using money as fuel for plot.
One of the things I'd like to highlight is that dungeons run by an Evil Overlord are pretty unpleasant places to work. Management are utterly uninterested in the wellbeing of their employees, and those employees, being nominally Evil, are much more interested in looking out for number one than engaging in acts of kindness, altruism or compassion. Cash is forbidden among minions (all coinage gleaned from dead adventurers is sent to Accounts and reassigned as loot for minions to drop when adventurers kill them), which means some alternative currency is needed.
The answer is favours.
If you want a fellow minion to help you then you either need to barter with them by offering them something tangible that they want, or promise to help them out in return - and you'd better be prepared for them to remember your debt. Maybe a favour will get you extra rations from the canteen, or let you avoid a shift mucking out the slime pits, or let you borrow a few heavies to teach a rival a lesson.
This favour-based economy pervades the entire dungeon, with minions trading their favours from one to another in exchange for, well, pretty much anything. The value of favours increases with the standing of the individual who owes it - so you might have shrewd investors who do favours for newcomers at Level 1 and call in their debt at Level 5 or higher, when the favour is correspondingly more valuable. Of course such speculators don't want to make risky investments, so they'll try to gauge how likely you are to survive before they'll help you out. Which in turn means it's very important to give the impression of having excellent long-term survival prospects, even if that's not actually the case. It's also very important to give the impression that one's word is one's bond - if you prove to be flaky or unreliable or reluctant to repay your debts when asked the value of your favours will drop precipitously.
My general inclination is to treat favours as money, perhaps even including the trade of I.O.U. tokens between minions. In effect each minion issues their own currency, with the value of that currency rising and falling as their fortunes change. Some might band together into groups, issuing and repaying favours collectively - their value is more stable, but not as likely to increase over time. For the most part the favour market is extremely volatile, heavily influenced by rumour, hearsay and speculation, with an emphasis on quick turnover of "portfolios" - the longer you hold onto a specific favour the higher the chances the issuer will get killed by some wandering adventurer, rendering it entirely worthless.
This could also provide plentiful plot hooks for PCs interested in playing the favour market - artificially inflating the reputation of someone who owes you so you can sell their favour to a third party for a juicy profit, or undermining that of someone whose assistance you need so you don't have to owe them as much in return; paying close attention to the grapevine to find out who's up-and-coming and whose fortunes are in decline.
My initial idea is to have the base value of favours be derived from a combination of (a) the difference in standing between two parties (by comparing their relative levels), (b) how much it helps out the recipient and (c) how risky or inconvenient the favour is to grant. This could range from trivial (covering their post for a couple of minutes while they dart to the latrines) to huge (risking severe reprimand for illicitly fiddling duty rosters in order to get someone out of a certain-death assignment).
This could lead to a pretty heavy bookkeeping burden (following who owes how much to whom and how much that's worth), so I'm inclined to save tracking specific favours for story-relevant interactions. Most day-to-day favour trading would be pretty low-level and uninteresting enough to just track as value of generic I.O.U. tokens on their character sheet.
If they need the assistance of a specific individual they might have to do their favour up-front, or make some kind of check to see if their hoard of favour tokens includes one issued by the individual in question, or do something for a bunch of other people to earn enough I.O.U. tokens to buy his services, or agree to let him call upon them for some unspecified task at a later date or sell that favour on to someone totally different - which makes big I.O.U. tokens a good way for the DM to send the PCs off on missions. Maybe it'd only be necessary to track NPCs who owe the PCs favours, as favours owed by the PCs can easily be traded off-screen and end up in unexpected hands.
It also means that at low levels the PCs can raise currency by running around doing favours for others, or even do very well for themselves by giving the impression of being bad dudes whose stars are on the rise - people will willingly do them favours with the expectation that they'll see a return on this investment when the PCs hit their stride - and if they owe an individual enough favours, maybe that individual will take steps to protect their investment. But this does mean that eventually those favours will need to be repaid, with interest.
Conclusion: Using favours as currency leaves a lot more room for story than a traditional copper-silver-gold fantasy economy - each I.O.U. token is a potential plot hook, whether the PCs are giving them out or accepting them as payment.
Who issued this token?
What was the original favour?
How did the token come to be in the PC's hands?
Where is the issuer now?
How can that be turned to the PC's advantage?
