If you are interested in the bean family, lentils have the highest amount by dry weight, don't need to be soaked, are cheap and don't cause that certain inconvenient reaction. Split peas also. TSP, textured soy protein is high protein, low carb, low fat and cheap also. Maybe the cheapest protein available when bought in bulk. Something like %50 protein by weight (raw beef is maybe %20). It's also fairly unpleasant to eat, highly processes, and helps support the mega soy industrial complex. It tastes a little like toast. You can add it to a fruit smoothie, but you end up with a strawberry and toast smoothie:eek:.
As far as complete and incomplete proteins go, most nutritionists now agree that you don't need to combine the protein foods each meal. In other word, the amino acids circulating in your blood from last night's chicken will balance out the lacking amino acids from today's lentils. Bodybuilders might disagree, but I'll assume you're not tying to look like Arnie S. in his younger days.