Like the mage certifications earlier, Frieren has lived long enough to see currency change dozens of times. She knows what it is, just has no concept of how much means what unless someone is asking her to pay for something she's buying.
That's why she relies on bartering goods she forages, and doing odd jobs for people (who then pay her in present value items and coins).
Frieren's sense of payment/reward for the work or quests she does gets explained eventually. I forget where.
Explaining it in meals is actually the concept of the Consumer Price Index. It's actually a great way to measure value.