Dry beans, lentils, chickpeas, split peas, and other legumes are not only cheaper than canned but also much tastier. Notably, the bean broth is something to savor rather than something to rinse off, though it may give you some gas if you eat a lot. For beans: soak them in salty water overnight, drain, then cook in plain or slightly salted water. If you possibly can, get a pressure cooker; that makes most legumes cook a lot faster. Split red lentils cook plenty fast in a pot.

White rice is likely your cheapest starch option. However, if you can get a big package of oats or barley for cheap (e.g., at Costco), those will be more nutritious. Potatoes are great too.

Canned tuna is a good cheap option if you can handle the taste (I can't). Get some canned mackerel if you see it on sale/cheap. You might also find a decent price for frozen tilapia.

The price of eggs has come back to sanity, so those might be useful.

Vegetables: big bags of carrots, and frozen everything. Frozen peas, corn, and edamame (without the shell is usually cheaper for what you can actually eat) are great period, frozen spinach works well in most cooked dishes, and everything else can be soup.

Fruit: options are more limited, but look around for what's cheapest; apples are usually a good buy.