Different ways of buying energy, and their costs.
A pound of rice (453g) costs about US$0.35, or 0.08 cents per gram. Each gram is about 5 kilocalories, or 21kJ, so in the form of rice, you can get energy for about 27 megajoules per dollar. If a 2000-calorie-per-day person got all their energy from rice (which would result in dying of kwashiorkor) they would use 60 000 calories in a month, or 12 kg of rice, or 26.5 lbs., which would cost US$9.27 at the above rate.
Vegetable oil is slightly cheaper as a source of calories, but it's in more or less the same range.
Protein is considerably more expensive; pure protein costs around US$7 per pound, but there are natural foods that are 25% protein by weight, such as pinto beans, which cost about the same as rice, with the remainder being carbohydrates. This means you can meet your protein needs without increasing the cost of your food from the all-rice diet above.
Of course, you need other nutrients to survive as well, but none of them in multiple-gram-per-day quantities, and none of them are things you derive energy from.
However! Both rice and beans require extra energy to cook them --- minimally, enough to heat them and a similar mass of water from ambient temperature to near boiling. If that's from 25C to 100C, that's 75 calories per gram of water --- figure maybe 100 calories per gram of food. But that's only 0.1 kilocalories, or an additional 2% "tax" needed on top of the energy in the food.
Around here, gasoline currently costs about US$3.00 per gallon. Each gallon is about 130 megajoules, so in the form of gasoline, you can get energy for about 43 megajoules per dollar.
Electricity in the US costs between US$0.06 and US$0.15 per kilowatt-hour, with an average just under US$0.10. A kilowatt-hour is 3.6 megajoules, so in the form of electricity, you can get energy for about 36 megajoules per dollar.
These costs are surprisingly much more similar than I expected.