[Nyororin]

As someone who cooks for a 3 member family each day - it is MASSIVELY cheaper to cook. Trust me on this. Our food budget for the month (not that we really have a set one, but...) would be gone in a week if we tried to pull off eating out and buying preprepared stuff. I`d have trouble thinking of things to eat that would knock the normal food up into the "prepared" food category in prices.
It is usually as fast for me to heat something as it is for me to make something - just a little more hand movement is involved.

Of course, if you have no kitchen things change, but yes - I have lived with a tiny tiny refrigerator before, and it can be done. Buy a tiny rice cooker and learn to make curry and some donburi things for starters. I`d like to see someone pull off eating for 3 days on 600 yen like you can with curry and rice. (90 for the half the curry base, 65 for a couple onions, 35 for a carrot, 50 for a couple potatoes, 150 for meat of choice... plus about 200 for the amount of rice you`d need from a bag.) The only ingredient that needs to be refrigerated is the meat. After you make the curry/stew/similar thing - you can keep it on the stovetop for 2 days as long as you thoroughly heat it every 8 hours or so.