Daily expenses in Tokyo?

Hi!

I'm planning to go to Tokyo in the end of August - September. Since I don't have much money, I would like to know the average prices beforehand. I can stay either 9 or 16 days and just don't know whether I'd be able to cover all the necessary expenses so I must choose how many days I can afford.

0) I'm a student so I can't just withdraw money from some ''reserves'', I don't have any. That's why I need to know the price of daily expenses and don't want to pay for unnecessary things.

1) I will stay at my friend's place. So no need to pay for lodging and for food (at least dinners since I guess I'll be out during the days).

2) I don't have to buy all I see in Shinjyuku or whereever. Well, we'll go to some famous places, a few arcades, at least once to karaoke and bars from time to time but I don't really need to buy anything there and can't afford to just waste money meaninglessly. So, some fun from time to time but nothing too expensive.
Well, and I'll do some cliché sightseeing for sure. ^^

3) Have to know the transportation fees. Should I just expect to pay 1580 for all-day tickets or it'll be less in reality? She lives somewhere in the border of Tokyo...

4) Did I forget something?
Already mentioned that i DON'T have count lodging and breakfasts + dinners and I DO have to count transportation, some not-too-expensive fun during the day and/or night and sightseeing. Probably some food from time to time during the day but it'll be very hot anyway so I don't suppose I'd be too hungry. Maybe some water...but I can't take something from home.

How much could this be for either
a) 1 day
b) 8 days (the 9th is just an airport)
c) 15 days?
Whatever will do.

I know I can just ask her directly, what I'll surely do, but I'd like to hear more opinions.

Thank you!

The cheapest way from Narita Airport to Tokyo is Yen 1200. A bowl of ramen for lunch starts at around Yen 500. A bottle of water Yen 120. Train or subwayride around Yen 130 to Yen 200, so only extensive travel justifies daypass. Some attractions in Tokyo are for free: Observation platform on top of Metropolitan Government Office in Shinjuku, Imperial Palace Eastern Garden, Mejji Shrine and Asakusa with its temples. Change your cash to Yen either in homecountry or at Narita airport, most banks in Tokyo are not used to exchange foreign money.

So, if I would expect to pay, let's say, average of 1200 a day just for transportation and occassional food and water, should it be alright?

Didn't notice that symbol for Yen cannot be displayed here... All the prices written by me are in Yen of course.

2,000 a day would be better.