You can do quite well nutritionally with 5 minute oatmeal, peanut butter, pinto beans, eggs, rice, potatoes, pasta and a couple of glasses of vegetable juice a day. Seasonings are a relatively cheap way to liven up this diet. I'd think $80/month would be reasonable buying store brands in bulk. Maybe even less.

There's usually a soup kitchen somewhere that would feed you a meal a day. I ate at one on a recent tour just for
fun. Good food, good company. I left a nice donation.

Don't forget dumpster diving. Now that is an art form.