When I was literal piss-broke, there was a college campus near me with an open food court. Couldn't afford the actual shops selling food there, but in that food court was a condiments station that randomly had one of those electric hot water dispensers for making tea, and styrofoam cups. It also had ketchup packets, saltine crackers, and pepper.
Turns out you can make a pretty passable tomato soup with ketchup and hot water. Bit of pepper and a handful of saltine cracker packets, and I had myself a hot meal for exactly $0.00
With some money to spend, rice is where it's at. Hitch a ride to Costco or Sam's with someone who has a membership, and they have iirc 50 lb bags of that short grain fortified rice for like... $15? That's well over 100 meals worth of rice.
Cook that up with literally almost anything that has some flavor or nutrients - whatever's cheap. Or just eat it straight... bland, but it'll fill you up. Eggs go great with rice.
Fair warning, you'll get fat. Cheap food is NOT usually healthy.