@Empiur I'll make an example, since you completely missed the point. In Milan a chicken breast cost between 5 and 8 €/kg; in Zurich, less then 3 hours away by car, it costs more than 15 €/kg. On the other hand, gasoline costs 1.5€/l in Italy, while it costs a little above 1€/l in Switzerland. I have no price point for mana crystals.
In our hypothetical fantasy world, chicken is sold only in big cities, since you have to barter in smaller hamlet, and it costs 5 big coppers for one oka, up to 10 big coppers if they kill the chicken and clean the meat at the moment of purchase. Bread costs 2 small coppers per measure, while mana crystals range from 2 silver to one regal depending on the size and quality. You are not able to find gasoline anywhere.
Are mana crystals cheap? Is chicken expensive? How many euro is a regal? How much is an oka?
The answer is: there is no point in the exercise, not even using actual currencies in our world, let alone in two completely different universe, one of which is magical (and fictional). What you can do is see how much money you can earn in a week of work and check whether you are able to afford an inn for the same time span.
Saying that one fictional coin has the same purchasing power of a yen presume not only that the fantasy country has the same production and distribution capability of modern Japan (and THAT is already bullshit), but also implies they have really similar culture and values similar things in similar ways. For example in Japan some watermelons are typically sold for more than 6000$: this is not because it is hard to grow watermelons in Japan, but because they have a strong culture about gifting, in particular gifting exceptional specimen of fruit. The same watermelon anywhere else in the world wouldn't sell for more than 20$, because no one would spend so much for a fruit, no matter how good it is.
In our fictional country, chicken will be easy to obtain in a village, since everyone breed at least a couple, while in the city it would be very expensive, since there is no preservation technology and chicken need to be kept alive in a place where open space is at a premium. Bread could be cheap, because grain remain usable much longer, or expensive, since the king decided to heavily tax bakers.
For all these reasons, and much more, comparing yen to fictional currencies in general, and using food cost as a reference in particular, is beyond stupid.