Children love cold pizza. You can pack a slice of pizza and add to the lunch box or pack whatever you like. Maybe a banana or a kiwi. Those are the cheapest fruits. You could also add some carrot sticks and some water to complete the meal.
If your children like fruit cocktail, applesauce and the like, do this instead of wasting your money on individual sizes. Purchase the biggest size that your children will be able to finish in a three day period and split it between individual serving size containers. Keep them in the refrigerator and pull one out for each child when you are making their lunches.
NOTE: You can use larger sizes and freeze the leftovers in small serving sizes to use throughout a few month period. If you do this with a few different products, then you can vary what the children eat each day.
I used to make my own crackers and peanut butter. I still do, actually. I would purchase the cheap store brand crackers, as well as the cheapest peanut butter that I could find. This usually meant round crackers, soda crackers and graham crackers. They are simple to make, and taste great.
Cheap Trail Mix: We eat these a lot. Even the daycare children love them. I make up a whole bunch, and store in a huge bowl in snack size zipper bags (which I later reuse as many times as I can)
1 bag oyster crackers
1 box animal crackers
1/2 to 1 box of cheap cereal
1 small cheap candy per bag
Just mix up the first four ingredients. As you are dividing them into baggies, just add a piece of candy.
Hope these help.