Ready Lunchboxes and supermarekt lunch is inexpensive in japan if you are coming from eu, uk or the us. Most supermarkets and grocery stores like AEON, Seven or Seijo selling sell fresh sushi during the day and have special lunch time offers. If you would like to sit down you can check out some of the cheaper sushi places similar to running sushi. If you are looking for dinner or high-end sushi, then you will have to book a reservation ahead of time or you can check out the higher business office building that have premium restaurants at the top floors.
Grocery Store Sushi boxes “nigirizushi” can be bought for around 1300 JPY plus. Premium Sushi Boxes have different composition of Fish, Seafood, Vegetables and Shellfish.
If you don't want to spend that much money on sushi but still want to enjoy Japanese Sushi - you can. Supermarkets offer sushi boxes starting from even around 300 JPY with fresh fish and vegatables.
Most places are per Plate instead of all you can eat in Tokyo, Japan. A plate with one or two pieces of Maki or Nigiri cost around 100 JPY. The price of a plate depends on what kind of topping you order.
If you order a ready made sushi set at a restaurant then it will cost around 1000 JPY to 2000 JPY. If you don't want to spend that much money then try to visit sushi restaurants during lunch time or at a 24/7 place during night time when they offer special priced menu items.