Have you ever tried to buy them without any money?
Characters are easier than people in that respect. They don't have to buy (or barter for) things in all stories, and often the money can be finessed by ever-full purses or the like but not always.
One sage soul said on a panel the problem isn't when you set a price for something in a story. It's when you have to set two prices. That brings the whole question of world-building down on you, relative pricing.
Of course, you could put them in different coinages. That's where I often scramble for something. Orderly coinage is an anomoly. You could count on coins of a country going evenly into each other -- mostly -- but not by the same amount; 30 of one into the next highest; and 12 of that into the next highest. And if there's commerce there's the possibility of paying by all sorts of currency. (Though money changers are the most likely.)
And then you have to name the coins.
And then if the characaters have enough to buy everything they need, you probably need to give them some way to protect it from theft.