Not necessarily. There are various fast food restaurants offering different kinds of Japanese food. Such as sushi, tempura, udon, soba, gyu-don, tonkatsu, ramen, Japanese curry rice, etc.
Go for conveyor belt sushi or standing sushi restaurant. Such as Kappa Sushi, Kura Sushi, Hama Sushi, Uogashi Nihonichi, etc.
Look for "Tenya". They offer Tempura rice bowl from around 500 yen~.
"Hanamaru Udon" and "Marugame Seimen" are the two biggest udon fast food chains. You can find pretty easily in Tokyo and other cities, too.
There are standing soba restaurants in station platforms sometimes. Apparently it is popular to busy Japanese business men.
Also there is a soba chain called "Yudetaro". You can find it easily everywhere.
Gyu-don is beef rice bowl. "Yoshinoya", "Matsuya", "Sukiya" are the three biggest gyu-don chain. You can have gyu-don from around 400 yen~.
Look for "Katsuya"
Ramen is usually under ¥1,000 everywhere.
- Japanese curry rice
"Cocoichi" curry house offers delicious Japanese curry on a budget. My Indian friend said he like Japanese curry from here, too!
I found that there were plenty of affordable options in Japan, much more so than in US cities. Mari-san did an excellent job of describing the various restaurant options, but even grabbing some onigiri or yakisoba-pan from a konbini while on the go was quite satisfying for me.