Same thing then, the jeweller's markup is just less. But we still don't actually care about it as it doesn't matter.
So I actually care about verisimilitude in my settings quite a lot, but I feel that often adding and defining extraneous detail does not help with that, it could even do the opposite. If we define things exactly, give them concrete numbers and such to be "realistic" then it actually draws attention to the matter and calls us to think whether these numbers are actually realistic and often they aren't. But if we leave things more nebulous, we can then just assume things will work in whatever manner is actually realistic, but we just do not know exactly what that is. Like the jeweller's business obviously works somehow, but we don't actually need to know his entire business plan and his bookkeeping to make that assumption.