Sorry, I didn't actually answer your questions. My favorite low-cost meal is muesli I mix myself in front of the grocery store, with rolled oats, walnuts, and raisins. Then a stack of tortillas and either cheese in mild weather or peanut butter when it's blazing hot. A bag of cashews and some more raisins for snacks. A pint of rich ice cream to eat while mixing the muesli. Fresh fruit and veg to fill up the bag and convince myself I'm eating healthy.

Basic food math; fat is 9 cal/g, carbs are 4 cal/g. A typical healthy mix is around 5 cal/g. To carry, say 4000 calories per day, you need about 800 g. You may need more than that. It can be a challenge to get that much food energy in semi-healthy food choices for less than $8.

For comparison, it'll take about 500g of tree nuts to get you about 4000 calories, and that'll cost $10 in most grocery stores. It'll take a kilo of oats to get there and you can get that for $4, but try eating a kilo of oats in a day! (Just kidding, do NOT try that.)

For further comparison, consider that a US gallon of gasoline contains 31,000 calories and I can buy that where I live for about $2. (When gas here hit a high of $4 a few years ago, a motorist pull up next to me and admired me for all the money I was saving on gas. When I told her, "You should see my grocery bill," she had an "aha" moment.)