I'm fairly certain that the Yen conversion thing in isekais is for the audience to have an idea of the prices of things in universe since it usually comes up once early and never again but it's still retarded since currency conversions don't really work like that, it's not as simple as:
'product a' costs X money
'product b' is similar but costs Z (local) money
Z must equal X money!
Banks do a lot of market evaluations between the two currencies to figure out how strong or weak X is to Z, how strong or weak is the market of the countries compared to each other.
100 US will get you more in Canada than 100 CAD will get you in America since the American bill is stronger than the Canadian bill.
(this is neither financial advice nor an accurate representation of financial institutions, just the ramblings of someone who is financially and mathematically illiterate trying to explain their frustration with a trope.)