all the way to eat McDonald's???...ahhahaah....

if you are on thrift mode...it's always great to find

a) late night suppers; sandwiches, onigiri, or bentos that you can reheat in your dorm for morning munchies. Mostly discounted over late evenings...so, best time to hunt.

b) also the not so healthy choice would be buying your noodle and cooking there.

c) on weekdays, there are some areas that you can see blue collar shirt worker going for lunch. Those areas are the cheapest of the cheapest..
I usually eat at $400-500 yen for an enormous amount of just buckwheat noodle, and a cup of occha.

Been there, done that...eventhough I have fixed allowance of Y$8000 per day just for food and misc.