It looks like Homnbu may have changed their process, I`m sure you used to be able to train there for a daily fee even if you weren`t actually a member of the Aikikai. Now it seems you need to be a member of the Aikikai in order to train. The fee to join the Aikikai is 8400 Yen and the daily fee is 1575 Yen. There are other options for weekly/ monthly etc depending on how long you want to stay. More infor can be found here under the Hombu Dojo "Guide" and "Visitors from overseas" sections
http://www.aikikai.or.jp/eng/index.htm
rgds
Bryan