For a cheap meal, do check out the departmental store food sections, where meals are avail for take away at a very reasonable costs. For even more savings, you can go to these departments approx 30min-1hour before closing and prices for these fresh items are slashed by up to 50%. On several occasions, I've scored a gourmet meal of sashimi for less than 3,000 Yen (prob 6-7 different plates of sashimi) during this period.