I like the ease of the decimal system for as the core baseline of the D&D fantasy economy. 1pp=10gp=100sp=1000cp.
Electrum is a natural alloy of silver and gold, so it fits right in the middle of those two values: 1ep=5sp=0.5gp
Because lore-wise for my campaign, I root the value of precious metal coins in metaphysical fact, so the value of coins doesn't fluctuate like the real world. Each of the precious metals used in coinage has inherent mystical qualities (applicable to magical endeavors) and therefore weight of the material denotes value, not economic standing.
All standard coins are 1/50th of a pound, no matter the material. However, the size of the coin may vary based on weight of the materials used, or if the coin has a hole or a different shape. A 1 lb. gold ingot or trade bar is worth 50gp because of the weight, because that is all that matters. For example: A real big non-standard gold coin/ingot that is 1/10th of a pound would be worth 5gp. 10 of those gold coins is 1 lb, which equals 50gp.
Civilizations only make coins for ease of economic trade, but it's not necessary. Merchants may use scales and reagents to weigh and test significant gold payments to make sure the material is legit. The gold coins minted by most hated, evil realm in the world are worth the same as anywhere else because the gold is the value, not the coin. That said, some coins from enemy realms are red flags to their rivals, and some rival realms melt coins from enemy realms and re-mint the gold in their own currency.
If a realm makes paper debt notes or ceramic marks, or other fiat currencies, those values may fluctuate just fine. They are just promises for real money.