At my local sam's club I buy ground turkey for 11 dollars and something cents for 5 pounds of it. I cook that up and split it into tupperware containers with veggies. Cheapest way I have found. I would suggest finding a store near you that sells things in bulk, such as a sams club, and check out the meat prices.
At the regular grocery stores chicken breasts are over 4 dollars a pound, and I can get a 12 pound pack for 1.97 a pound at sams club. Milk, eggs, ground meat, peanut butter, and large economy sized bags of nuts are some of the cheapest ways to get the calories and protein in.