I think all the fast food places have some sort of value menu. You might not get exactly what you want, but it will still appease your appetite.
McDonald's has their $1 Value Menu, which is just small portions of their food, fries, hamburgers, etc...
Wendy's has a $1-1.40 menu which is probably the best. You can get a 5 piece chicken nugget, fries. baked potato, chili, or a side salad. If you do a nugget, chili, and salad, you get a pretty big meal for under $4.
Burger King also has a value menu. I am not that familiar with it though, I'm sure it's your typical burger and fries. etc...
Even Krystal can be cheap: their burgers are under a dollar and the chicken sandwiches are like $1.50. The Chicks fill you up. so 2 should be enough plus fries.
I personally would go to Wendy's. I love their chicken nuggets and at $1, you really can't beat that price. I never get sauce either, I think they are so good on their own.