Beans are cheap protein. My favorite is black beans, so easy to use in so many different ways. In soup. Refried. Salads. Eggs are pretty cheap too.

The veggies are what is tough, especially in off season.

We make a lot of soups with a variety of different bases, from tomato to curry. Beans can be from mung dal to more standard fare. Then we really load it up with veggies. You often can find collard greens that are inexpensive, even when organic. (I am not overly enamoured of the benefits of the organic label and I emphasize local and small operators over organic). Frozen spinach would work too. Or frozen beans. I also usually throw in some fresh corn if available and if not, frozen. Sometimes we throw in meat for flavoring, like chorizio sausage. It ends up cheap because we use very little.

Spices are your friend. Curries to taco seasoning flavor our soups.