Cheap & Tasty Eats in Tokyo

Yes, foodwise, you can get by in Tokyo for very little. Lots of eateries offer filling meals for ¥500-¥1000 (S$6-12). Fresh and tasty enough for the price.
http://www.metropolis.co.jp/tokyofeaturestoriesarchive249/237/tokyofeaturestoriesinc.htm
http://www.tokyoessentials.com/eats.html
http://smt.blogs.com/mari_diary/2005/09/how_to_eat_in_a.html
http://www.chowhound.com/topics/379801
http://35degrees.com/article/103/cheap-eats-in-tokyo

I can vouch for good old Tenya, which I remember kept me well-fueled on a cold winter's day. There's one outlet close to Nishi Shinjuku Hotel, on the main road! Man, I would love a bowl right now!

Mos Burger is also worth a try. The last time I had it, it was so much better than in Singapore.

Yoshinoya is everywhere too, but the taste is well, so-so. Maybe depends on the outlet.

Matsuya is the other ubiquitous chain. They have a vending machine that dispenses tickets for the meals you choose. Curry, donburi, set meals, hamburgers, even breakfast sets. Wow, the website menu even lists the calories!