I usually spend about $100 per month, but I have a roommate who is a serious gardener; he encourages me to help eat the various veggies from the backyard. I also visit a few food pantries regularly (you have to shop around to find the best ones in your area, but if you find them, you'll get a lot of basics for free.) One has a really good selection of canned veggies, beans, cereal, and basic pasta, and another has just-about-to-expire bread (which is good if you have fridge/freezer space).
My actual grocery trips usually consist of seasonings (onion, garlic, herbs, etc), tea, specific ingredients I like that pantries don't have very often (mushrooms, olives, artichokes, couscous, Nutella, beers), and the occasional potluck/party contribution. Roommate buys enormous amounts of eggs, and has no problem with me eating them as well (it's better than eggs going bad), as well as milk, butter, and similar ingredients I don't use often enough to justify buying for myself.