You did say cheap and not healthy.. but still
Eggs, cheap, but too much cholesterol for the protein you get IMO
Milk, So much Lactose would probably kill your stomach and guts
Tuna is OK, but with more than a few cans a week I'd be worried about Mercury
Pasturized Egg whites are clean but not sure how much they are per gram
Bulk frozen Chicken Breast would probably be your best cleanest bang for your buck IMO, other than the good standard mail order Bulk Whey concentrate.