If I can live with 2000 yen a week, you can survive with 1000 yen a day. Just go to your nearest 100yen store and pick out 9-10 food items. That should last you at least a day, maybe 2 if you buy in bulk (As in those big liters of water, an entire loaf of bread, etc) and plan out your meals accordingly. Here's a sample meal plan:

1) Breakfast: A piece of bread (Or multiple slices of white bread) and something to drink (210 yen)
2) Lunch: A pack of instant noodles and 1-2 rice balls (315 yen)
3) Dinner: A pack of white rice w/ pack of curry, a fruit of your choice, and something to drink (420 yen)

And you still have 55 yen left over