I couldn't do $400 on a regular basis. Yesterday DH and I went shopping. We spent about $250, there are only 2 of us and the food might last 1 week. Now, if I were really cutting corners, I could possibly do a month or so for 2 people for $400, but eventually I would splurge on something!
I will offer advice on chicken nuggets. Make your own. It's cheaper, tastier, and more nutritious. I cut and pound chicken breasts, coat them with egg and seasoned (I season them) breadcrumbs and fry (I don't deep fry, just in the pan, but you can do whatever you prefer). To save more money, use stale bread for breadcrumbs. You can do the same thing with dark meat.
Also, I never bought chicken parts when on a budget. Always a whole chicken on sale. If you take off breast meat that's 1 meal. Leg meat is another meal. Pasta, rice and beans and salad are your friends. Save the bones and wings for stock. Meal number 3. So now one chicken (I assume cost of around $10 just got you 3 meals for 4. None of these calculations will work if you're feeding teenaged boys, but it's fine for normal appetites.