Just randomly punched in BOS-KIX for Oct 10-17. Routing thru SFO on P fares for $5400. If you change to BOS-TYO and click the "my schedule is flexible" for 6 days you get $5287 pricing. It'll cost more if you choose certain days of the week, but not that bad... As high as $5787.
But of course I'm assuming that you are looking to fly out of BOS.
Google flights has a $3031 fare if you do two stops on the return.