Quick meal to save your time!
Cooked beef over rice (牛丼-Gyudon) is one of your reasonable food while you are traveling in busy Tokyo.
One regular sized bowl only cost about 400 JPY (≒4 USD / 3 EUR)!
It is cooked with soy-sauce, sugar, dashi (soup stock). You add chili powder if you like spice.
I personally recommend to put picked ginger on the side.
Here is the link to major franchise gyudon chain.
Shops are located near the train/subway station.