Well mexican is the cheapest by far. If you cook up some rice, beans and meat with spices ahead of time, throw it in a bag and bring some tortillas you can make burritos for next to nothing.
Also PB&J eggos are the shit- toast two eggos of your choice and then use them to make a PB&J. The little squares hold in all the goodies and the crispyness from being toasted helps keep it from becoming mush.