Barbecued beef or pork sandwhiches are pretty cheap for how much you get. Buy a roast of some sort, throw it in a slow cooker, add a bottle of bbq sauce (maybe half bbq half steak sauce), a tablespoon or two of vinegar, a cup of celery (optional), half a medium sized onion (optional), garlic+salt+pepper to whatever degree you like. Cook 3-4 hours, pull roast apart with some forks and pull out bones if there are any, stir it up, then cook another hour.
I think that's basically it, although I don't have my cookbook on me. I eat on that for at least a week every time even when I try to share as much as possible.
Edit: Salads are really cheap and good too.