When we move between economies/powers, we must convert our money from one currency to another. Several factors influence conversion rates, including differentials in inflation and interest rates, account deficits between countries (how much they owe each other), public debt, trade terms, and economic performance. Does this sound like something an audience wants to read about, or something we want to determine for not one, but two fictional sovereign powers? Probably not. It’s reasonable to desire getting it “right,” but as is often the case, many details impact this and no one from our fictional world is going to show up and say we’re wrong.
Do we need to show conversion and rationales in our work? Not usually. If we’re doing a story with sovereign powers having just risen or fallen, or other dramatic changes within our tale, then it’s obvious that currency could be disrupted, especially for units of value backed by the government, but most of us can skip it. We don’t typically know why our own dollar is rising or falling and audiences certainly won’t understand what’s happening on a fictitious planet, especially if we don’t tell them. Explaining can actually get us accused of having done research and then dropping it into our narrative.
The conversion arguably matters less with units of weight, like gold, because its size and rarity don’t change. But it might be differently valued in one place. The exchange is typically transaction by transaction and decided between merchant and customer, as opposed to units of value, where the exchange rate is set by the sovereign power or other governing body and changes by the day (on Earth). A merchant must abide by it, and so do we and our characters. It’s simplest to have a character express a reaction to how far their money is going and not focus on details. Being consistent matters less than other subjects because rates change daily anyway.
