First off: people don’t swap them in their diet, a better exemple would be wheat or soy - which are what the 0.99/p chicken eat [edit: and it's closer in term of nutrients].
Second: the shelf price you mention includes gouvernement subsidies and economy of scale. The grains price should be the one paid by the fermer, adjusted for smaller packaging. Your comparaison may stands where you live because of political choices and societal evolution. It doesn’t in a more liberal and non regulated juridictions, does it?