what are the cheapest healthy foods?

- Dry beans.

- Rice and probably some other grains.

- Peanuts in the shell.

- Canned sardines (with the bones).

"Superfood" is mostly a marketing term imo, and anything advertised as a superfood is sure to be expensive (and healthy, yes, but not to the extent of performing miracles). The price of fruits and veggies varies greatly with location and time of year, so I don't know which ones count as cheap. But if you're eating mostly the things I listed above and only drinking water, you'll probably have spare money for some veggies & fruits even though they're not very cheap, and then your diet will be balanced (rice + beans is a complete protein, peanuts and sardines provide fats and more protein, and the veggies will provide the micronutrients). But some "superfoods" can be found for free, for instance dandelions are all over the place for much of the year (plus they're organic).