non-trivial currencies

A lot of game actions involve exchanging some currency for some other currency or resource. Or, okay, depending on how reductive you want to be you could say that every action is a currency exchange, but sometimes the currencies are complex difficult-to-evaluate concepts like "a good position". But often they're just numbers of a thing - gold, credits, sheep, beans, whatever.

Thinking about different ways in which currencies can operate in real life might give some ideas for ways to make these exchanges more complex. (You don't necessarily want to add complexity everywhere, this is just something to consider).

OK first, look at international currency exchanges. Relative values change over time, which is already interesting to play with: decision-making under uncertainty to try to get the best deal. Shopping around for deals - different money-changers may offer different rates. Maybe there's a fixed fee per transaction so there's an incentive to change a large sum in one go. Obvious complexity.

More subtle is the complexity that can arise within a single currency. Typically we like to imagine you have just one number and bigger is better, but there are different conveniences. Cash vs. EFTPOS: some places won't accept cards; carrying cash is riskier; possible transaction fees. Different denominations of cash; buses which don't give change so you'll overpay if you don't have the exact amount.. basically these are like different currencies that are cheap to change between (but not free, because time has value).
Or someone sends me a cheque and it's such a hassle to actually get around to banking it (plus if I'm in Germany what am I even supposed to do with this they don't use these here anymore).
And since these things are instantiated as physical objects there are different values based on physical properties. Notes or cards weigh less than a bunch of coins, so they're easier to carry but harder to bludgeon with. The raw materials composing them have a value - if the value of a coin's metal ever exceeds the face value of the coin there's an incentive to melt it down and reuse it. Or there can be particular value in specific units - like the giant robot vending machine that only takes quarters (so we traded with a beggar; notes more valuable to him for coins more valuable to us).
Also collectors - someone might value a particular coin more highly because it completes a set for them, by denomination or year.

And hey there's complexity even in directly comparable numeric values, looking at how much relative difference they make. When you're broke finding a note lying in the street affects your situation way more than if you're doing okay; once you can afford everything you need increasing that number further just becomes a way of keeping score.

Some of these situations might be useful to think about when designing games? These types of consideration are used in board games sometimes but rarely in videogames - putting things on a computer makes it easy to treat everything as a number and think no more about it. Just something to think about using maybe I don't know!