For good and cheap lunch, try Depa-chika (literally means 'basement of department store'). It's a food market, usually located at the basement of most department stores. You can find them anywhere around major train stations. Many shops offer free food sampling, some shops sell 'bento' lunch boxes to go in a reasonable (JPY500-1000) price, and usually there are food court with fast-food stands, too.

Kaiten-sushi (sushi-go-round) is another option, where you can pick up from variety of sushi plates from the conveyer belt. Plates have different colors according to its price, so you can keep your budget as you go. Green tea is usually comp, but miso-soup cost JPY 100-500, which you have to ask in person. This is somewhat lower-end in the hierarchy of sushi restaurants, but still maintaining much better quality compared to most Korean/Chinese sushi bars in North America. Bon appetit!