It can be difficult to determine how much anything costs in a fictional world, or the wages people are paid, but this is easier than it seems. Why do we care? Because we may want to show a fantasy character, for example, paying one silver piece for a drink and then three gold pieces for a dragon, and then we wonder if that’s on target or ridiculous. There’s a simple trick for this: use Earth values from the country where you live, then tweak this.
For example, if I buy prepared food, I know how much things cost in the United States. Using some made-up numbers, let’s say fast food is under $10, a cheap sit-down dinner is under $20, a nicer outing will set me back $30 for just myself, and something above $50 would be expensive, a meal for a rare occasion like a holiday. How does this help?
We’re not going to use the word “dollars” or “cents” in a manuscript set on another planet, so let’s say I’m writing fantasy and have decided that the generic “coin” is my “one dollar.” A turkey leg or steak-on-a-stake (i.e., fast food) might cost me 5 coin, a sit-down meal at a tavern catering to warriors and other working types might run 20 coin, a nice inn or restaurant might set me back 30 coin, and if I’m doing the latter on the eve of a major holiday or buying the most expensive thing on the menu, I’m spending 50 coin. Notice how the numbers are the same as my U.S. analogue.
I don’t have to determine value. I borrowed the economics of modern America as a starting point, at least. We can change the numbers, adding a zero so that what’s five dollars here is fifty coin there. Or we can multiple or divide by three if we want less relation to our source. We can do the same with wages, products, and services. For something not in use on Earth anymore, substitute an item that is (italics) in use. Not sure how much a two-handed sword would be worth? Well, it’s a pretty big, specialty weapon, so research big, specialty guns (still in use on Earth) and compare them to more ordinary guns to gauge a price. Plausibility is the bar to get over, not being “right.”
Don’t use the current price of a wagon, because who is using one aside from the Amish? Like cars, wagons come in different sizes, so create a range of values just like cars have. Note that unless a character will buy or lose the use of a wagon in our story, or express pride or dissatisfaction with one, pricing this is unnecessary world building. On the other hand, it adds believability if a farmer goes chasing after our main characters after they’re stolen his best one, which cost him a year’s earnings, unlike the older, more dilapidated ones. It can also make characters seem less like a jerk when they steal the latter instead.
When something is fictional, we must decide how much value it has. Even if dragons are common, one may be the equivalent of a military jet with fantastic speed, electronics, and weaponry. Those cost millions a piece. This means only a government’s military likely has them, but this analogy has a flaw: it costs money to build jets, so is a naturally occurring, living possession like a dragon worth less? Undoubtedly. Training of that dragon, rarity, and possibly equipment like a saddle are the only actual costs (other than lives) of taming one. The cost also doesn’t matter if they’re not for sale. If you want a dragon, you might have to kill its owner, assuming the dragon lets you or allows you to take ownership. But dragons are a unique subject.
What about something less spectacular, like a flying horse? This would be the most expensive horse, so determine the value of horses (like everything, they come in a range) and raise it. Someone who trains such creatures is unique and likely earns far more, and belongs to a higher social class.
For trading systems, it can be difficult to determine that one knife is worth three cows, for example. There isn’t a simple way to determine this, but we can infer that cows are naturally occurring and take no special intervention, whereas a knife, especially a well-made one, does. Therefore the knife is more valuable than a single cow? How much more? Story needs are a good way to determine this; if a character badly needs the knife and to keep the three cows, then this is a pain point for him. Do we need that? A pitfall to avoid is showing characters consuming or using items that aren’t local in the absence of trade with other communities. This is a minor detail, but avoid showing an isolated place that has every item available in a city, for example.
We don’t need to get value “right.” Our invented world can have gold as common as rocks and therefore gold has no value. Supply and demand means we have significant leeway and only need to be consistent, which matters more when writing a long series in the same world (or region of one). To achieve that, it is best to base our system on Earth values and change the numbers in a consistent manner (such as multiplying everything by three) and using different terms. Besides, even on Earth, rates differ from one place to another. A townhouse in a nice neighborhood might get $500k in one locality, while the money buys a large single-family home with yard and pool in another. No one from our planet is going to show up and tell us we’re wrong. All of this applies to SF as well, except that we have even greater leeway due to tech that has never existed here. No one can say how valued it would really be.