American Airlines just introduced rntp fares for Osaka, Japan traveling Nov1-Dec15 (purchase, I think, by Sept 15) for around $600 rntp + tax, depending where you travel from US gateways are DFW and ORD (and LAX?)

Great mileage run...for sure! check aa.com for details.

This is comparable to what a european mileage run would cost, but you can potentially earn a lot more miles. For instance, DFW/KIX is about 6900 miles one way (14 hr flight westbound, probably about 12 hrs eastbound)