Nutrition-wise it's hard to top breakfast cereal and milk in regards to cost. Cereal has increased in price--as has milk--but the two combined could net you several meals worth for about $5 if you buy off-brand.
Dinnertime meals? Hamburger Helper Beef Stroganoff is delicious, but high in salt, at about $5 with ground beef. Provides two or three meals for one. Also, instant rice combined with lower-cost fresh veggies--i.e. celery baby carrots, onions--and a small can of chicken breast can make a nice, healthy and satisfying meal that could last a few days for one at about $5 or $6.