With more potassium than bananas, cheap white
potatoes are a surprisingly good source of vitamin C and B6. Some vegetarians will eat potatoes BBQ style as well as cabbage "steaks" and carrot "bacon."
My equation for cheap and nutritious:
(Sweet potato OR white potato OR rice) + (beans OR green veggies, such as spinach, broccoli, brussel sprouts, cabbage, frozen peas with carrots)
I will also make an Asian noodle bowl with veggies such as cabbage and broccoli. I also like corn on the cob, with just salt or pepper instead of butter. I also like collard greens cooked with kale and/or mustard greens.
Bananas are my favorite fruit cost-wise. Bananas are a great source of the soluble fiber
pectin, and they are part of the BRAT diet (bananas, rice, applesauce, toast), which is good for preventing GI symptoms, such as from GLPs.
Oats are also cheap, with a great, unique micronutrient profile. Like @Tattoo85 , I will sometimes put frozen blueberries in my oats (or in my cereal or yogurt).