This would be the time to cash in on your miles, if you have enough. UA and NW have Y/C/F awards for 20/30/40K (F on UA only), which are excellent values.

If not, once in Japan, you could go to one of the discount travel agents (they have ads in English language newspapers such as the Daily Yomiuri, Japan Times, or in the free Tokyo Classifieds), which should have GIT tickets for at most half of the figure you quoted. GIT tickets are group tickets which must be prepaid (usually by cash), picked up at the airport, and do not yield full miles on many Asian carriers; they tend to be cheapest on UA and NW anyway.