Apple One is available in Japan but only if the iCloud Storage Plan attached to Apple ID is 200GB or less. If you have the 2TB plan, Apple One is not available. This makes sense because Apple One is supposed to be cost effective and signing up with Apple One Individual or Family with the 2TB iCloud Storage Plan isn’t cost effective. Apple One Premier isn’t available in Japan but I think it will be around the 2,500~2,600 mark when, and if, it arrives. Here’s my scenario:
Apple One Premier would be worth it for me just for iCloud storage and Apple Music for the family. The other services are nice extras. Until then Apple One isn’t cost effective for a 2TB iCloud user like myself, at least not with the current pricing and configurations. Your mileage may vary.