Around 300 US going over Shanghai with China Eastern Airlines. Minimum time seems to be around 7 hours with changing in Shanghai
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EDIT:
I just found a cheaper option
Fly from HK to Singapore and from there to Kuala Lumpur both flights with Jetstaraisa. Than take Malaysia Airlines to Narita.
This will only cost 180US but im not sure if its worth the time (and you´ve got to book/coordinate 3 different flights).
Also this price was for mid april, it might be more expensive if your schedule is thight.