I just came back from a college trip over in Tokyo, We stayed for just over a week, and excluding flights and accomodation, I managed to live on £200 (which gave me 45,000 yen) and still had a bit (though not much) for tourist spending and buying gifts etc.. So it depends on exactly what you want to do. I'm not sure it is the most expensive for everything, I think London is! Eating out was actully quite cheap, as a group of 17 we practically ate out every night, averaging just 2000 yen each. Having a Japanese friend did help. I think going to the less touristy places means prices drop a bit, though I'd learn a bit of japanese as most of our waiters didn't speak english (plus polite to anyway, and very usefull, you only need some basics). Travel can be a bit confusing, as diffrent companies own the trains, luckily, if you get confused you can always fare adjust to each station, transfer. I think I spent about 5000 yen on travel, you could probbly go cheaper by sticking to subway and not taking JR, but not too sure.
Of course, you will need a lot more money if you want to buy presents to bring back!
Have a great Time.
One more think, have a look for weekly mansion hostels, we stayed in one for just over a week sharing with four in each apartment for 23,000 yen each. we stayed in Kiba, which is just outside the centre of tokyo, quick to get into on the metro. Not actully a bad place, I was expecting a small room, but turned out to be quite a reasonlable size place.