Beans and Lentils.
Both are super cheap, and high in protein.
For a good meal/snack...boil a whole bag of lentils, then drain the remaining water. put a few tablespoons of butter over them, a few tablespoons of EVOO, a healthy dose of the following: Cumin, Aleppo pepper, Curry powder, Ancho Chille Powder, Kosher salt, black pepper. I would also recommend browned hot sausage thrown in there. Its awesome.
Dried beans are cheaper than canned, you have to soak them though. Beans and Rice ftw, put a bunch of different spices in there and go to town.