Optimal would be in-season local vegetables, in-season local fruit, and remaining calories from a variety of grains (and legumes) and occasional varied inexpensive meats.

You could make it cheaper with frozen vegetables, but you’d lose some nutrition (and taste if you did care), and by skipping fruit (losing some nutrition) and meat (again losing some nutrition)

Nutritionally, dried fruit is pretty ok if it’s not sweetened. Canned fruit is pretty worthless, and juice is worthless.

Canned vegetables are fine if cheap, but lose some nutrition over fresh. Fermenting in-season vegetables can preserve most nutrition to tide you over for when nothing is affordable.

Most calories would be from grains and legumes: lentils, peas, rice (brown has more nutrition, white is usually cheaper), beans, corn, etc. Whole grain breads are nutritionally great if they aren’t full of preservatives. If you dont have a local baker just skip bread altogether.

Avoid coffee, beer, wine (probably), cider, liquor, smoking, and drugs. Tea might be fine but it has no nutrition so it might also be avoided.

If you can afford it (and enjoy it), meat is very nutritious and calorie-dense in moderation, so a small reduction in starch for a proportionally small increase in meat can be beneficial for some lifestyles. Obviously you dont want to reduce fruit or vegetables since they have the most nutrition per calorie in general, but a diet exclusively of fruit and vegetables is expensive and unreliable (and possibly not nutritionally optimal). The type of meat depends on where you live: shrimp, anchovies, chicken, goat, beef, whatever is cheap and available.

Some spices, oil, and salt would make it all a lot better tasting, and wouldn’t add much to the cost. This is pretty much the diet of working people all over the world, just with different specifics.