I've come to look at foods in basically two categories - ingredients and processed foods. These are my own categories, so please don't take them as official.
To me ingredients are ones you can eat as is or by simply cooking them. This would be fruits and vegetables, grains, pastas, meats, eggs, cheeses, milk and other dairy products.
Processed foods are the ones where the ingredients have already been put together to form something like a cookie, a loaf of bread or a frozen meal.
I've noticed something. Bread made from ingredients costs less than processed bread bought at the grocery store. The individual ingredients for my meat loaf (which I'm making tonight) cost less than a family size Stouffers frozen meat loaf dinner or the pre-made, uncooked meat loaf at my grocery store that is ready to cook. Homemade cookies usually cost less than packaged cookies.
When I buy ingredients instead of processed packaged foods and make my own foods, it costs less and I have much more control over the quality and earth friendliness of the ingredients. Right now, I've got a huge pot of chicken noodle soup on the stove, and at least half of the ingredients are organic. Others are all natural. This whole pot is costing me about $20 to make (I've doubled the recipe).
Making your own food from basic ingredients may cost you time, but buying processed packaged foods for the majority of your meals costs you more in money and quality, and it costs the earth. Take a look at the ingredients in a frozen dinner. They can come from all over the world and by the time that frozen meal hits your table, it's much better traveled than you may ever be.
Even if you can't get all of your ingredients locally, buying ingredients instead of processed foods will still minimize your food miles. Add that to the fact that your food will be better quality and cost you less, and you can see why it's a better choice.
I know not everyone can cook from scratch every night of the week. I can't. But several nights a week, I make sure I organize my time so I can do it. If you never do it, try it just once this week.
A little hint - the more you do it, the less it will cost you. Once you get used to cooking from scratch and become familiar with favorite recipes, you'll be able to buy more and use an ingredient in more than one dish. If a dish calls for half a red pepper, diced, you can also plan to have kabobs that week and use the remaining half a pepper on the skewer. You'll be able to waste less, saving even more money. That type of planning takes time to learn, and I don't have it all down yet, but once in a while, I have a week where it all comes together.