With $20, peanut butter would not be on my list, no kidding. DO you know what a jar of peanut butter goes for? I LOVE peanut butter, and since it is such a nice protein, feel free to eat it in place of other things I really don't care for. But it keeps going up, up, up, and now they are saying there was a bad peanut crop, and it will be scarcer and higher.
I would buy rice, because it is filling, and can be used as the base of many things. The tiniest bit of meat, some veggies, and a lot of rice makes a good meal. One you would pay $$$ for at your local Chinese restaurant. And although in some places it is expensive, I just bought a 20 lb bag for $7.99 on sale at the most expensive grocery in town.
People throw away too much food when they cook. The tops of celery, potato peels (where the vitamins are), onions because they dried out a little in the fridge (and yet, we buy dehydrated onions, duh). If nothing else, make soup.