This is an example of why we need to make DIY D&D the norm in this hobby. I have seen, far too often, DMs dismissing an idea or mechanic or game element because they don’t like how it works when they instead could've hacked it to unlock its true potential. How many DMs don’t even bother with Alignment or Encumbrance or Darkness just because, as they exist in 5th Edition (and throughout the editions, generally), they kind of suck and need some reworking? I’ll show you how to do that right now.
In D&D, currency comes in three common denominations. Ten copper pieces to one silver piece, ten silver pieces to one gold piece. Beyond that you can have ten gold pieces for one platinum piece, and of course the infamous one, five silver pieces to one electrum piece AKA two electrum pieces to one gold piece. Electrum is the one that breaks the nice, consistent pattern. It is exceedingly rare. It seems pointless. It feels like if you received that as your treasure from the DM then they were trying to troll you a bit. So most people don’t use it and that’s that.
That’s the lame way of doing things.
Contrary to popular belief, electrum is actually a real metal and not just a fantasy thing like adamantium or mithril. It’s the naturally-occurring alloy of silver and gold, and was used as the standard for coinage in much of Antiquity before metallurgical processes were advanced enough that silver and gold could be refined separately into their purer forms. What this tells you is not just that it’s halfway between silver and gold, but more importantly that it's the money of the ancients. This is what you find in ruins and crypts and the castles of old giants and the hoards of elder dragons. And here’s how you change the rules to make it 1) easier to use in your game, and 2) interesting to use in your game.
Forget the ½ gold thing. Now, instead, electrum has the exact same value as gold. Or rather, it has the same value as whatever your standard unit of currency is. In my setting it’s silver, so 1 electrum = 1 silver in my world. If you were playing Knave then it would be 1 electrum = 1 copper. For most of you it’ll be gold so I’ll stick with that. Electrum has the same purchasing value as gold but… it doesn’t share the same buying power. That is to say, electrum is only accepted in older markets where people still deal in it (e.g. giants, dragons, the undead, demons, maybe elves, etc.), and in those markets they will not accept gold. Likewise, when you return to your home village and head to market, no vendor or merchant will accept payment in electrum (except maybe for an antiquarian who is buying the electrum itself off of you).
So, what does your game now gain by using this version?
The world is now divided in two. There are two world economies that are functionally separated. Your PCs now have two financial standings to maintain. You can be wealthy in one country and could be simultaneously impoverished among a foreign population. It helps the DM still grant the PCs treasure as a reward but control the group’s total potential purchasing power nonetheless. It is, marginally, more realistic for representing the possibility of a scenario where your money might not be good for anything when you walk in far-off lands.
This works particularly well for my setting, because it’s based entirely off of the dichotomy of the Overworld (where humans live in the light of Heaven) and the Underworld (where everyone else in the setting lives and from where all things came originally). The further down you go, the older the societies you find. What better way to reinforce this binary split than with the mechanic of two currencies that don’t cross over? The way a lot of DMs use the Underdark, they fail to leave an impression of it on their players any greater than “basically just some caverns.” But this houserule makes them truly feel like two different worlds that you’re passing back and forth between. And it also helps me thematically speaking, because I want to create an association between the Underworld and a sense of history and antiquity and genesis.
A rich man on the Overworld can suddenly be a poor man in the Underworld and that places PCs into a weird set of circumstances to work through. Depending on if you’re the kind of DM who makes their own item lists for when the PCs go shopping, you may already have been using a separate Underdark one from your standard one (since it makes sense that they’d have different items or different prices). I like using this divide as well, and I would say that, rather than shifting prices up or down, it’s probably better to keep prices mostly the same but just change from gold to electrum or vice versa. The PCs can get a consistent understanding that a Potion of Healing costs 50 X no matter where you are, so they can instead focus on getting enough of the type of money they need.
You can still extract the same amount of potency out of it for your campaign even if you aren’t doing an Overworld/Underworld thing like I am. If your setting ever has a far, far away country or another plane of existence or an alien planet or whatever, you can steal this trick and make it feel that much more distinct gameplay-wise by giving it a different currency the Players are forced to use. I firmly believe that worldbuilding in RPGs is best done through gameplay, which means that not hacking the rules of gameplay will inhibit your game from escaping the default world of D&D when your game could instead be set in your world.