I shoot for 3, no more than 4. I prefer real food, but if for some reason I run out of chicken or tuna, then I will go up to 6 a day (while gritting my teeth).
It's really not that pricey if you buy your protein in bulk. I probably spend about 15 bucks a week on protein, not that bad considering that is about the same dollar amount as 2 super sized extra value meals from McD's.