I am not familiar with the specifics, but when I lived there last year the cheapest nvmo plans would offer 20gb data for ~2-3k yen/mo, while main infrastructure carriers would be 2-3x that, but recently Rakuten has become rich enough to just build out their own network, and I assume combined with government pressure, within the last year they have all rolled out value plans that are around 20gb for ~3k yen.