Egg whites. I buy the pre-made and just have a pint straight with a huge amount of veggies mixed in (whole pan full to a couple layers deep).
chicken breasts: Use the George Foreman and grill the pack of 3 at once. cut them in half. then use on salads, in sandwhickes etc. Each half breast is 100 calories and about 20g protein. Very convenient for the math!
Carbmaster yogurt from Kroger: 12g of protein, 80 calories and only 40 cents! ( I buy about 30 every few days!) Actually take my shopping cart and shove it against the cooler (with opened door) and just shovel them in. maybe I should ask about getting cases from the back!
FF feta: almost pure protein (not sure if cheap)
FF cream cheese: almost pure protein (not sure if cheap)
My Kroger also has some very cheap defrosted fish (less than 2 dollars per pound).
Personally I don't really buy the "need to have 1+ g of protein per pound". There's a lot of disagreement on that and it seems very possible that the RDA types might be right. I do have a bit more than RDA (usually 150 per day, but if I'm down to 100, I don't sweat it). So maybe just decide to have less?
There's also a good novel called King Rat by James Clavell. Give it a read.