The season where most vegetables and fruit are cheaper is fall and winter, as the temperatures go down the more common vegetable ripen.
Most Japanese people buy every night what they intend to eat that evening or the next day. One of the reasons they do this is because electricity costs, as well as the high price of a large fridge, are more expensive than it's worth. The downside to this "buy now eat now" mentality is that nothing fresh is meant to be stored.
How to eat for cheap.
As an exploring person, I set out to find out how much it would cost to feed me for a day buying only from the 100yen store.
This is the result of my research. There are prepackaged pouches containing curry or pasta sauce which are available. You can also buy 500g of dry pasta or a small box of precooked rice to be heated up in a microwave for 100 yen.
I bought a box of tea bags, 25 tea bags (too strong for me, should have gone for a green tea).
A large apple pie/chausson, with a cup of tea it was a great breakfast.
I also got a bag of chips (Taco flavour) which were a great midday snack.
Finally, I got a pouch of curry (240g). It wouldn't win any culinary awards but it was fairly decent. It's also enough for two small meals.
Total for a day of food including the rice I bought 420yen. Price of tea for a month 100 yen x2? If I were to eat this stuff everyday I'd spend 12600 yen on food a month, or about $130. Not bad!
And if you're wondering, the total calories was about 300 for the curry pouch, 300 for the apple pie and 350 for the chips. The rice I ate with it probably added another 400-600 calories. I also noted that most of my day's vitamin requirements were taken care of. Hmmm... Maybe I should start a new diet fad...