So, what the heck is that "rea", huh? The guard said the entry fee is one large silver coin. Btw, a large silver coin for entry must mean that silver isn't worth all that much in that world, so there's plenty of it. Anyway, the guard also didn't know what on Isekai is a "rea", so he instead referred to the physical coin. The innkeeper, however, suddenly did talk about that mysterious "rea", so it's a real thing. I suppose we can assume one rea is the coin with the least value. Possibly a small copper coin. If it is, copper must be plentiful enough to be worth very, very little, so one would need to pile 500 copper coins (one rea coins) in front of the innkeeper to spend a single night in his establishment. Not overly convenient.