Just a quick recipe for tonight, my version of waldorf salad for lunch, makes enough for two people (or two meals for one). This is quick, light, nutritious, and cheap.
I start with two apples, usually one granny smith and one fuji or pink lady, pick whatever you prefer (about $.66). Wash them, then core and chop them up into bite-sized pieces (I leave the peels on). Crush and add about 1/4 cup of chopped or broken walnuts (about $.35), mix together. Then mix about 1/2 a tablespoon of mayo and 1 tablespoon plain yogurt, a little more if you used big apples (about $.05). Stir the apple and walnut mix into the yogurt/mayo dressing. I usually chop up and add about 2 to 3 ounces of white chicken meat as well to add protein (about $.31). Serve on slices of whole wheat bread (about $.41).
Total cost, including chicken and 4 slices of whole wheat bread is about $1.78 for two meals, or about $.89 per person/meal. It has a full serving of a fruit, contains around 350 calories per person/meal, about 20 grams of protein each, and can be relatively low in the bad fats (this depends on the exact brands you use).
Leave out the chicken, and you cut the cost to less than $.75 per person/meal. The calories will drop to about 260 per person, but this also cuts the protein to less than 10 grams per person. You could also skip the whole wheat bread, but this would probably only be enough for one person (and you'd lose the fiber, protein, and b vitamins in the whole wheat bread).
The plain yogurt can do a lot to reduce the amount of fat when you substitute it for most of the mayo, and I think it adds a nice tang to the dressing. I do find it needs a little mayo still, to have the right flavor. The walnuts account for a lot of the fat and calories, but nuts are supposed to be especially good nutritionally, and it wouldn't be a waldorf salad without some. I do serve this open faced. If you made the filling into regular sandwiches, you could get four full sandwiches from this amount (double the bread).