Did anybody mention cabbage yet? It's one of the least-expensive green veggies, in season. Healthy too. Boil it, or eat it raw as a salad. You don't need to put mayo with it!
And for those in rural areas, how about harvesting from nature? There are naturalist groups that do "wild edibles" hikes and will tell you what to collect, and how to safely do it.
Do NOT eat the mushrooms, though, unless you really know what you are doing!