I think it's around 850 Yen = $10. And yes, you just create an account that is tied to the Japanese region/store, and you can buy their games. Then switch back to your own account. Keep in mind though that you lose things like saved up Nintendo coins when you switch stores.
Anyway, I burned some gift cards on Ring Fit Adventure. It's actually rather fun, and it makes for a surprisingly good workout! Not a big fan of a bodybuilding dragon literally flexing on me though...
