Just keep your eyes open for good deals. I actually found Tilapia for 1$ a pound in 4oz fillets! I bought 40 pounds of it. (Beats the hell out of the normal 5$ + a pound price)
Lately I have been eating a ton of brown rice, brocolli, and tilapia. I make 1cup brown rice and 1/2 lb tilapia, and 2 cups brocolli and that is around 1000 calories, basically no fat, 40 g protein from the fish and more from the brocolli, and the actual cost of that meal is just slightly higher than 1$. (Cheap fish rocks!)
Eat alot of eggs as well, and be creative.