The reason that chicken is so much cheaper is because of the steroids (growth hormone in the food) This makes them butcher size very quickly. there for making room for more chickens in less time and not having to feed as much food. Also chicken food itself is very inexspensive.
Chicken parts have less nutritional value then whole chickens and whole rats. It is true that many snakes eat bird prey in the wild, But we are not talking about wild birds or wild snakes.