Carrots, potatoes, celery, onions. Pound for pound they're much cheaper than McDonald's double cheeseburgers.

Edit: Next time you have $20 or so, get a 5 lb tube of ground beef from FoodMaxx ($10), a 10 lb sack of potatoes ($5), a bag of carrots ($2), a bag of celery ($2), a 5 lb sack of onions ($1.50), and some aluminum foil. For each person eating, chop a carrot, a celery stalk, half an onion, and a potato. Spread half of this vegetable mixture on a square of foil. Put a half pound of ground beef on top. Salt and pepper the beef (free if you want to gather complementary packets somewhere). Spread the rest of the vegetable mix on top. Wrap up in the foil (so that no food is showing), put the bundle on a cookie sheet, and high broil in the oven for about 45 minutes. Unwrap and enjoy. We called them Hobo Packs, and they're delicious and very filling.

You can make 10 Hobo Packs for your $20, and that's just until you run out of beef. You'll have a lot of vegetables left over.