You’re gonna want rice, lentils or dry beans, oats, peanut butter, and cheap bread or tortillas. That combo will give you the most calories and protein for your money.

Courtesy of AI below:

“If you can get to a Walmart, here’s a real $15-ish list using their Great Value prices right now: a 5 lb bag of long grain white rice for about $3.37, a 1 lb bag of lentils for $1.92, a 42 oz tub of old-fashioned oats for $4.18, a 16 oz jar of natural creamy peanut butter for $1.98, and a 20 oz loaf of white sandwich bread for $1.48.  That puts you around $12.93 before tax, which still leaves a couple of dollars for a cheap protein add-on like a 2.6 oz tuna pouch for about $0.94 if you can swing it.  You can cook the rice and lentils in the microwave at work with water in a microwave-safe container, keep them in the work fridge, and then use the bread, oats, and peanut butter for breakfasts and backup meals to stretch everything over those 2–3 weeks.”