Stuffing

I'm not on a budget and just casually gaining, as someone mentioned on this thread, pasta. I lot of super market chains have the boxed macaroni and cheese, which sell for a $1.00 and even less, makes almost a family size portion. I binge on it every so often to feel really stuffed, you might try it.

The supermarket is honestly your best bet. They usually sell cheap hamburger patties and bread. I usually find 6 pack of burgers at around 2 dollars with bakery buns at a 1.50 or so. They sell cheap microwavable pizzas also.
You can also buy long loaves of bread for cheap and fill it with every cheap meat you can find cheese, salami, ham, swiss, sliced tomatoes and tons of mayo. Hell even some cheap breaded chicken patties you can get plenty. I've made two 2ft breaded chicken sandwiches with veggies and cheeses for 4 bucks.
Including the bakery section you can buy day old donuts for less then 2 dollars a bakers dozen. Cakes, pies, pastries a for cheap I usually walk out with 2 stuffing setions worth of food for 20 bucks.