A while ago I looked at saving food for emergencies and determined that the cheapest way to get calories and protein would be peanut butter. In the comments people pointed out vegetable oil and sugar were also very cheap. At the time I thought eating just peanut butter, oil, and sugar would taste disgusting and make you sick, so you'd need to spend a bit more and get some variety. It turns out, however, that fortified peanut butter is one of the standard things to distribute during famines. Apparently they're sufficient to act temporarily as a sole food source for a month. Ingredients:

• non-hydrogenated vegetable oil
• sugar
• peanuts
• nonfat milk powder
• whey powder
• maltodextrin
• vitamin and mineral complex
• hydrogenated vegetable fat
• soy letcithin
• mono and diglycerides

They taste like peanut butter.

(I got to taste some because people at work are raising money for an organization that, among other things, distributes these. Unfortunately the posters they've made treat the cost of a month's worth of food-packets as the cost of saving a life when most of the people who receive the packets would not otherwise have died. Because the packets are $0.50/each and a month's supply is $45 this contributes to the problem where everyone is immersed in wildly inaccurate numbers for the cost of saving a life.)