Stick to basic, whole foods. The more refined they are, the more they cost, the less you actually get out of them and the faster they leave your belly (and the sooner you get hungry!).

Bear in mind that nutrition doesn't have to be balanced in every meal or even every day. Many of the things we need we only need "boosters" on once a week, every couple of weeks, maybe only every few months. When you find stuff on sale get more of it--by the time you get sick of it, there'll be something else out there that's cheap and different. And the more creative you are with recipes, etc., the better. There's a fair amount of anecdotal evidence to suggest we actually do best using seasonal foods--which works out well if you've got access to local farm stands/farmer's market, things like that--stuff in season is generally more plentiful, hence cheaper, and because it's fresh, it's also more nutritious.

Use more expensive things as condiments. Make sure everyone gets sufficient protein and then fill up on the least expensive, lower carb veggies you can find--a little fat makes everything taste better

Also, and this sounds strange... but make sure everyone in your household gets enough water and sleep, since our bodies naturally demand more food to compensate for lack of sleep and fluid.