In terms of calories per acre, corn is absolutely king. Chickens and pigs turn corn calories into meat calories at a ratio unreasonably close to 1:1, and feeding people without meat is very difficult.
This is why meat is expensive and why people in less developed countries can't afford as much meat as Americans.
But I also imagine it can't be cheaper to buy X food and feed it to chicken to then eat the chicken compared to just eating X food.