In terms of just pure quality for the price, this is the best place to go. It's a small standing sushi bar that is very highly regarded by the locals. Pieces of sushi are from ¥100 to ¥300. If you don't know what to choose, just ask the chef for an "omakase" (leave it to the chef) and he'll serve you an assortment of excellent in-season fish. You can also have drinks for about ¥300-500. Most people pay less than ¥3,000 here. Located a 2-minute walk from the North Exit of Nakano Station.
Sakae Sushi (榮寿司)
If you're looking for a quality sushi bar at a cheap price, then Sakae Sushi should probably be at the top of your list as well. You can get, for instance, a nigiri sushi plate which includes uni (sea urchin) and ikura (salmon roe) for just ¥1,360. They have one item in particular that Japanese customers really rave about, and it's their "saba bo sushi" which is a mackerel rod-shaped pressed kind of sushi. You can get 8 pieces of that for ¥800. There is also a cheap drink menu. One cup of sake starts at ¥300. Sakae Sushi is located a 6-minute walk from the Nakano Station's North Exit.
Sushi Zanmai (寿司ざんまい)
Sushi Zanmai is a sushi restaurant chain you can find all over Japan. It's conveniently opened 24h, every day of the year. The restaurant is spacious too, so it's easier for a large group to walk in. They have sets that are around ¥2,000 and full courses that come with a good-sized Japanese soup for ¥3,000. For a reliably-good, reasonably-priced sushi, Sushi Zanmai is an excellent option. It's located a 3-minute walk from the Nakano Station's North Exit.
Kaisen Misakiko (海鮮三崎港)
Kaisen Misakiko is a famous kaiten sushi restaurant chain. It has two locations in Nakano. The sushi is nothing outstanding but the prices are cheap; plates start at ¥110. You order from a touch screen, which can be set in English, and the sushi comes from the conveyor belt to your seat. Both locations are in the Sun Mall, at a 3-4 minute walk from the North Exit.
Ganzozushi is another pretty good kaiten sushi restaurant that achieves a good balance between price and quality. Some of their items are as cheap as ¥90 a piece. They have seasonal specials that you should really try when you go there. For instance, when it's in season and in abundance, the fatty tuna is sold below ¥100 a plate. Another thing to try is the "kobore ikura" (¥500) which are salmon roe sushi pieces that overflow with topping (that's what the name literally means). Located a 4-minute walk from the Nakano Station's North Exit.