One of the “easier” ways to create a unique world is to choose a different form of currency.
It’s something people notice, since money is ubiquitous.
The issue is that money is fairly well developed.
It needs to have certain features, or else it flat out doesn’t work.
Examples
In Warhammer 40k, orks use their own teeth as currency.
Since every ork has access to teeth, there isn't absolute poverty.
Since the teeth decay, hoarding teeth isn't feasible, and it means that orks need to constantly try to expand to get more teeth.
Since every ork gives teeth in a tax to their boss, it means that war bands constantly expand and fight, giving the combat happy bastards yet another reason to go to war.
A huge amount of fantasy universes use copper, silver, and gold as currencies.
Metro uses ammo.
In real life, we see many alternative, and ineffective, currencies, ranging from company script, to cryptocurrency, to hyperinflationary national fiat currency, to precious metal based money.
Background
The issue is that all of these are fundamentally flawed in some way as a currency, rendering the economy of that location extremely vulnerable to various shocks that would rightfully upend the entire economy.
A currency fills three major roles
1. A medium of exchange.
2. A store of value.
3. A unit of account.
A medium of exchange means that it is accepted by enough people as having value to be used for trade.
Rather than needing to find someone who wants your goods to trade in a chain for something you want (like in every Zelda trading questline), you just give them money and they give you the item.
This is why money is more efficient pure barter.
It acts to lubricate transactions between peoples.
A store of value means that you won't see all your wealth disappear if you don't spend it now.
Which means that you can save up for major purchases, you can make deals that last for years (like mortgages), and people can actually retire on what they’ve earned over the course of their life.
A unit of account means that you know the value of the money and it is standardized.
Imagine if the only form of money was in fine art.
You could exchange a Van Gogh for a house, and a large spiky suspended ball for a car.
Art could fit as a medium of exchange and a store of value, but actually trying to compare artwork to artwork would drive people insane quickly.
You'd be in the situation that a dollar isn't worth a dollar, or one Van Gogh isn't worth another Van Gogh.
There is no way to convert between lesser and more valuable pieces in a logical manner.
Now, why is that relevant?
Because a huge amount of monetary systems in fiction fail these requirements and allow for overt exploitation or unduly hamper the government's ability to respond to threats.
Problems
With regards to the ork teeth, what is functionally happening is constant hyperinflation.
Since the teeth decay, there is explicitly no store of value.
Which means that the only orks who can afford the best and most fun toys are the warbosses and WAAGH! leaders.
There are probably billions of orks who just want to save up for a spaceship or motorcycle or set up a Squig farm of their own, but will never be able to because their money falls apart before their eyes.
Somewhat more seriously, for a race dedicated to war, constantly decaying teeth means that the number of war bands that can attack space based shipping or otherwise need more complex and expensive equipment is limited, reducing the race's overall effectiveness in combat.
By attempting to be clever with inflation, by making it so that it couldn't happen, they created the effects of hyperinflation.
And, since it is still a money based system, that means that a race designed to go to war can't do it as effectively as they should.
In WoW or D&D or any of a dozen universes where wealth is metal based, using multiple metals as various values of currency would have a similarly debilitating problem.
It destroys the unit of account.
Basically, the government sets an exchange rate between the chunks of metal, making gold 10x as expensive as silver which is 10x as expensive as copper.
But the rarity and expense of gold isn't 100x as much as copper.
It is usually much much more.
So, it makes counterfeiting extremely attractive, since you can produce 100 small value coins, of the actual metal, and exchange them for a coin of much higher value.
Or if it is in the other direction, where you can exchange something where the face value is less than the value of the metal, all the government is doing is funding a small extremely active and profitable metal reclamation industry.
This would be an ongoing and unavoidable issue, one that could cripple a government attempting to keep enough money in circulation, or cripple business if the government failed to intervene in an ongoing manner.
Metro has the same issue of lacking a unit of account.
The value of a bullet depends on what you're facing and what weapons you have.
Even if the nominal value of a .50 cal armor piercing round is high, the number of people who can use it is very low.
Consequently, you'll see the value change and possibly invert, as use brings more common rounds out of circulation and makes the more expensive rounds increasingly obviously useless.
Without a set value across the board, or something interchangeable and universal, the currency itself will always be in flux, making for a really really shitty form of money.
And a fairly cursory read of human history reveals why being inventive with money is a bad idea.
Company script is money that doesn't function as a medium of exchange.
It acts to tie people to a small location and punishes merchants, intentionally gimping economic power of consumers.
Bitcoin, aside from arguably not working as a medium of exchange, fails as a store of value.
It is inconsistent and disconnected from reality, making any long term contract in it unfeasible.
It has many of the same problems as hyperinflation, except you don't know which direction the value will go.
Less common now, but currencies that are based on the weight of an amount of precious metal suffered from failing as a unit of account.
As gold coins were chipped, sweated, plugged, adulterated, or otherwise debased, the value of the coin and the face value became disconnected, and a buyer was dependent on merchants being trustworthy with their scales.
Functionally, money is the way it is because it works fairly well, and the obvious alternatives tend to fail in overt ways.
Attempting to be clever with monetary solutions isn't really feasible most of the time.
Solutions
So, are there any currencies that actually make some degree of sense in world, and aren't just "GOLD FOR ALL"?
Surprisingly, yes.
Fallout's bottle caps have surprisingly good arguments around why they are used beyond the water traders of the Hub.
Basically, becoming a medium of exchange is more based on mutual consent than it is on logic..
Shells, pieces of wood, large rocks, feathers, and shiny metals have all been used.
Ragnar Benson, of the survivalist fame, claims to have found isolated African tribes that were using Austro-Hungarian bills in the 70s.
Unless there's a government that forces something, pretty much anything can and will be used.
By selecting it as a currency, the water traders turned bottle caps into a representative currency, each cap was a certain amount of pure water.
They gave it some base level of value that was universally accepted.
Outside the Hub, people were willing to trade for them since they had value, prompting other people to accept them on since they could be used in trade, gradually shifting it to something like fiat, abet unbacked by a government.
Fallout has a surprising amount of trade across the US, where jet reached the East Coast and the Wasteland Survival Guide reached the West in a couple decades.
Over 100 years, it's completely reasonable for bottle caps to become an accepted medium of exchange, valued because people value them.
With regards to unit of account, bottle cap or not is pretty effective.
And, since it doesn't have higher denominations, which could introduce the potential for arbitrage, it works.
Abet annoying to count out hundreds or thousands of caps of you had to do it manually.
For a store of value, after 100 years as an accepted currency, most large stashes would have been found, and the only input would be through Nuka cola, which is more valuable as soda than caps.
And, as described in game, without a press and marking machine, counterfeiting is difficult; labor intensive and involved.
There really isn't much way for more caps to come in, which preserves its value.
The greatest issue with bottle caps is long term deflation as the population expands, but, while the wasteland continues, population growth will be muted.
Consequently, caps in the Fallout universe ought to provide a stable bedrock for longer term business and functioning governance.
Assuming that the world’s inability to actually rebuild despite that being the story for hundreds of years gets resolved.
So what?
So, what makes a good fictional currency?
Well, that’s mostly fulfilling the functions of a currency.
- Medium of Exchange – that can be nearly anything, as long as it is universally accepted. Attempting to create a new currency for each trader, like some sort of munted script, would be horrible and useless.
- Store of Value – The currency should not be easy to counterfeit, which implies 2 things. Either that it is nearly worthless on its’ own (like paper currency) or that the value is derived from a hard to fake commodity, like gold. At the same time, making this needs to be difficult, or else you have the issue of the Elder Scrolls with Transmutation and turning iron into gold, which is also the foundation of their currency. Hyperinflation means broken economies.
- Unit of Account – If you’re going to have more than one currency, you need to directly tie them together. More money should be based on the same features as the Store of Value, either just a bigger number on the front, or a larger chunk of hard to adulterate or change money.
And, if you think you’ve solved a major problem, you really really should talk to an economist before designing your world around a special feature.
