Tuna (in water) is probably the cheapest form of protein (after all, that WAS the original question right?). I mean, ounce for ounce, pound for pound. Plus it has Omega 3's which is also a plus. But don't go gorging yourself with it at every meal. Just like with anything, excess is not good.
Having said that, whey protein isolate is also another inexpensive source if you're looking in the right place. NOT GNC. NOT some Super Duper Max Gainer Miracle Supplement X5000 - I would STRONGLY recommend against these proteins powders with Arginine added, Creatine added, etc. etc....NOT because those added components are bad, but because the QUALITY of ingredients AND the QUALITY of the ingredients are just horrible. Remember, these are BUSINESSES. Companies. What matters most is the bottom line. Profit. Money. Period. If they can get away with using less or lesser quality ingredients and still have the ingredients label say "contains X", then they will do just that.
If you want creatine in your protein drink, then add it yourself. If you want arginine or ornithine, then add it yourself.
My nutritionist has opened my eyes to so much in this category. With changes in my nutrition ALONE, I have dropped 6 percentage points of body fat, shaved .5 seconds off my 40, and increased my standing vert by 3 inches. It really is amazing what the body can do when it is fueled properly. And remember, "Fueled properly" means via nutrition FIRST AND FOREMOST! Supplements ARE NOT a miracle and will only work properly with a proper nutritional foundation (hence the word "supplement" aka "in addition to" not "instead of")
I get all my stuff from Purebulk.com - NO I'm not an affiliate and I don't get any profit from promoting them. It's just awesome pricing for pure ingredients.
Enter coupon code fffitness for 5% off. Again, I DO NOT get anything for that. They love my sports nutritionist and have hooked him up with that code for his clients as a courtesy. I don't think he gets anything out of it either. I've asked him before and he said it was cool to share the coupon code so......check it out for yourself.