Our breakfast routine includes oatmeal we make from steel cut oats. We buy them in bulk at a local health food store.
Last time we bought some we tried to get enough to last a month or so, and it cost less than $2. This got us to thinking about how cheaply one could live on oats alone.
Steel cut oats cost 49 cents per pound at the health food store. According to Quaker, every 40 grams of dry oats yields 150 kcal. If you assume a 2,000 kcal/day diet, then the arithmetic works out to 1.2 lb/day, which is 59 cents per day or $17.64 per 30-day month. I guess the water and energy to cook the oats might cost something, but that would be pretty insignificant. We cook ours in a crock pot, which is pretty efficient.
So basic sustenance like this costs about $18 per person-month. Even less if you buy the oats by the 20 lb sack instead of by the scoop. This is certainly not a nutritionally balanced diet; multi-vitamins would be a real good idea, and there are probably a bunch of other nutritional deficiencies you'd have to worry about. And I'm sure unflavored oatmeal gets gross fast when it's all you ever eat. But it's reassuring to know that in a desperate situation we could stave off starvation for $36/month.
This also gets me to thinking about our own grocery costs. We cook a lot of recipes at home, and use a lot of the usual tricks to keep the costs low. But even with those tricks, we spend a lot more than $36/month. I had thought of our grocery spending as spartan, but in absolute terms it's luxurious.