I want to know the mindset of any fantasy author who needs to convert their fictional currency to IRL currency. Because it's not just one or two, it's far too many and it just doesn't make sense.
As long as what a copper/f-rank/bottom tier adventurer makes (on average) meets the cost of the cheapest sleeping arrangements and 2 meals, the rank existing makes sense and you don't need any kind of comparison. Not even in Isekai.
If the strength of the effect of the isekai requires currency comparison, it's not worth calling it an isekai. Just have the main character come from the world to start with and save whatever dignity you might still have as an author.