I'd say sandwiches, depending on what you want to put in them. A loaf of healthy (low sugar) bread isn't going to be the cheapest option on the shelf, but if you're dividing the cost by the number of sandwiches you can make out of it, it still ends up amounting to a large number of really inexpensive meals. I normally just add some meat, cheese, lettuce, and tomato, and it's very nutritional and also delicious.
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