Price of things in Japan are really funny, some items say a 15 year old bike or engine is described as 'junk' and hardly rates any bidding interest. Then some items fetch sky high prices only to reappear a few weeks later for the same price.
Vintage mxers are thin on the ground and prices are high. The KX would be around $6-7000 say once you paid for the bike, local shipping, export and agent commission(you can't by directly from Yahoo Auctions). You would have to get it shipped by sea in boxes to avoid gst and customs.