Doggett saidChicken breasts aren't that expensive.
This. I buy boneless, skinless chicken breast in bulk for $1.99/lb. I live on the stuff. I have fish once or twice a week plus the occasional steak.
Beware of canned tuna. The cheap stuff, while providing lots of protein, is loaded with sodium. Most of it is canned with 'chicken broth'- read the labels.
For carbs, oatmeal and brown rice are cheap. Buy fruit & veggies in season at a farmers' market if you can, otherwise some of the frozen varieties aren't bad. Just make sure that they're not loaded with sauces and gunk.
Nuts and peanuts are a good source of 'good' fats and calories. I buy unsalted in bulk because I have to be very careful with my sodium intake.Read the labels!