Kaplods had some great practical advice for cheap + healthy. If you're really tight, forget the fresh produce - it is far too expensive to justify when you are really worried about just subsisting for a month until things get better. Frozen veggies are often on sale and are cheap even without sales and are perfectly viable nutrition-wise. Dry beans are far cheaper than canned, and if you're pressed for time do as kaplods suggested and make ahead/freeze. (Did you know canned beans were the first convenience food?

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Crock pots can do wonders with cheap discount cuts of meat - things that would usually be tough and unpleasant become delicious. Easy to toss together a dinner if you have a crockpot, it can also do dry beans.
Oatmeal is dirt-cheap and generally better for you than boxed cereals. Plus you can customize it every morning to keep it different so you don't get tired of it. Boxed cereals are never going to be as good of a deal as plain rolled oats bought in large quantities.
Have you visited Hillbilly Housewife? Her website is full of very frugal, easy meal ideas and has "emergency" menus to feed a family on very little for a month. She does try to keep an eye on health content as well, though I admit a lot of her stuff tends to include boxed/white flour items because they are just plain cheaper. She does a pretty good job at finding a balance, though.