I think something you should keep in mind is the fact that just because foods are cheap (such as prepared foods) it doesn't mean they're going to be healthy.
While Lean Cuisines and prepared mixes are fine to have sometimes, you really have to watch the sodium content as many of these foods are full of it. While you may not have a sodium issue (like hyptertension or heart disease) now, eating too many of these could lead to major problems down the road.
I buy these often, but in moderation. I make sure we're eating almost entirely fresh food rather than prepared or frozen entrees. Frozen veggies and fruit are another story though--use them all the time, buying when they're on sale.
I am a firm believer in sales. I always get high-fiber breads, frozen vegetables, fruits, and meat on sale to freeze. I also look for cheaper cuts of meat, it takes a little longer to get them done sometimes, but marinades are powerful in helping with tenderness. In reality, there are many cheaper cuts of meat that I have introduced my husband to that he actually prefers now to the others he used to have that were more costly.
I also always save on cost by cutting up my own veggies and fruits, the precut ones don't seem to ever be as fresh and you're paying for someone to do the work for you that really doesn't take that long.
I also buy whole chickens and de-bone and skin them myself as they are almost always cheaper by weight.
I am also trying to lose weight and the best success I have had so far is by incorportaing more veggies and fruits into my diet. They seem cheaper in the long run, have fiber, minerals and vitamins and are very versatile. I usually look for recipes for them on vegetarian sites. They have some neat, unexpected ways to serve them that I just love!