Some ideas from Quality Health

Tips for Eating Healthy on a Budget

Eat at home. This may be the single best way to eat well and keep your food costs in check. Yeager says the average U.S. family spends about 45 percent of its family food budget on meals prepared outside the home, 80 percent more than making the same food at home.
Use basic convenience foods. Canned tomatoes and other staples can be the foundation of a healthy and inexpensive meal. For example, the University of New Hampshire Cooperative Extension demonstrates that you can make a cheese pizza by buying the ingredients and assembling it for less than $3, compared to paying about $9 in a restaurant.
Give up bottled water. Bottled water is more expensive than tap water, and manufacturers have spent a lot of marketing dollars convincing us bottled water is healthier than tap. That point is questionable, but what's not debatable is that bottled water is 240 to 10,000 times more expensive.  Read more here.

When you also consider the environment, there are even more reasons to avoid over-packaged convenience foods.