Wow, I had no idea ISI had dorms. I applied through a swedish institute which placed us in guesthouses (56 000 yen each month in rent, so quite cheap and nice with all the Japanese people living there, but also 40 minutes from school by train).
Anyways, I think people here are sorta confused about grocery prices... either they haven't lived in Tokyo, or their own country has pretty sick prices for groceries. Personally, I found groceries to be really expensive in Japan. Eating at home was almost always more expensive than eating the same meal out. If you bought groceries late when the prices of fresh wares are lowered, it becomes economical... but still, you also have go buy food really late.
Of course, some stuff is worth making at home. The kind of food which lasts for a few days.