Where can I find cheap sushi restaurant in Japan?

There is a 100 yen sushi restaurant in Japan.

1. Genki Sushi or Uobei

In genki sushi you can eat sushi for only 100 yen and they also provide seasonal menu. I recommend you to try their autumn seasonal menu. I think it is the best of all season. You can check here for more information: http://www.genkisushi.co.jp/en/ 

2. Kura Sushi

This restaurant is also offers 100 yen sushi. Personally, I think this restaurant have a smaller portion than Genki Sushi. However, the sushi is fresher than Genki Sushi. Here is the link to the website: http://www.kura-corpo.co.jp/en/