I should not say “cheap packed lunches” but “free packed lunches” since many of us throw out the leftovers of our meals. So, next time you tidy your kitchen after diner, think twice before throwing your leftovers in the bin (or giving them to the dog). Use them in your packed lunch at work. Remove bones and fat from meats and cut the flesh into smallish cubes. Re-use your leftover pasta and rice too. Add a few more ingredients to bulk it up if required. It does not take that long. For ideas, have a look at my post about how to make varied packed lunches.
The one meat I find does not work well cold is lamb. It is too fatty and leaves a small white covering which is not pleasant to eat when served cold, in salads for example. I am not keen on cold cooked root vegetables either but this is just a matter of taste really. I like cold cooked peppers in salads but I think raw is best for your packed lunches.