Soy meat, flax seeds and peas are probably the best known such sources. Are there other cheap sources of protein?
Chick peas/Garbanzo beans, lentils, and other legumes (black beans, Great Northern beans), and nuts (nuts are a bit pricier). Cheap and vegan/vegetarian friendly!
If you use every bit of a whole chicken it becomes a significant value. The meat can be eaten as a main, but stretched even more by being shredded and used in dishes such as chicken pot pie, enchiladas, quesadillas, and so many other dishes that use some chicken mixed in with vegetables. Once the meat is off the bone the bones should be used to make stock. You can freeze portions of stock that can later be used for soups, stews, and other recipes. If you subtract the cost of canned chicken stock or broth from the cost of your chicken, you will find that your much-tastier stock saves you about half the value of your chicken if not more.
Another option for inexpensive meat is to shop the sales and look for manager's specials - discounts on meat which will expire within the next day. Yes, you've got to cook your manager's special meat immediately, but a quick braise in the crock pot might give you a base you can freeze for a future meal.
Egg whites are a pretty good one. Our local grocery store even sells them in milk carton containers.
Turkey is one of the cheaper meats - here in the UK anyway. I'm not talking about whole turkeys around Christmas or Thanksgiving time but I know that in my local supermarket turkey mince is significantly better value for money than say beef or chicken.
Eggs are cheap. But they do have a lot of cholesterol in them.
While cheap is good, health is more important. One thing to watch out with vegetarian proteins is that you are getting what is considered a "complete protein", ie. contains all the essential amino-acids.
I believe quinoa is a complete protein source. It is also possible to mix a few non-complete proteins such as rice and lentils to obtain a complete protein source.
Tofu is a great source of proteins, it's very cheap, but doesn't taste anything. I suggest to cook it with anything tasty you like and the tofu will get all the taste. Ex: throw some tofu in a pad thai = full of cheap proteins and really tasty.
Quinoa is an excellent source of protein.
- It's good for vegetarians.
- It is also good for people with allergies to gluten b/c it is gluten free.
- Culinarily I love the texture and body of the grain. Think of it like couscous but with a little thin shell around each grain.
- It also comes in different colors (white, red or black) so that you can use that to your advantage when creating a dish.
You should try mung beans! They're delicious and small (lentil-like) - just wash dried beans and then cook them for 30 minutes in boiling water for an al dente like texture. Then you can toss them into anything you like - 100g has 30 calories and nearly 8g of protein.
I add them to canned soups (when I'm feeling lazy) or saute them with ginger, garlic, chilies, and tomatoes when slightly motivated.
VERY cheap and good.
In The Netherlands, they are promoting insects as food. I haven't tried, but they must be cheap and proteins. Not vegan, though.
Oats are actually a pretty decent source of protein. Wikipedia (via the USDA nutritional database, which unfortunately does not have a linkable URL for that data) lists their protein content as about 17g per 100g. Note that this is still not as high as most beans, which tend to be up in the mid-20s.
The Wikipedia article also mentions that Oat protein is nearly equivalent in quality to soy protein, which has been shown by the World Health Organization to be equal to meat, milk, and egg protein. I'm not sure what measure of "quality" they are using; I would guess that it might be related to the portion of the full complement of amino acids that constitutes "protein"; soy is well known to contain all of the requisite compounds.
So if oats effectively constitute a "full protein", then they may be a more, or equivalently, economical source of it than common beans which need to be mixed with rice. Note that this is not because rice is more expensive, but because the "protein" proportion of rice is much lower (wikipedia says 7g/100g). Honestly, I suspect that all of this stuff is a bit more complicated than these statistics make it out to be, so please take this with a grain of salt, unless you happen to have a degree in nutrition and/or biology.
They are also generally about as cheap as food gets, at least in these parts. They run about $1 (CAD) per pound or less for lug-free quantities (about 3lb), and they are very compact, absorbing about twice their volume in water. Pretty much the only thing cheaper around here is potatoes. Which are . . ridiculously cheap, but only contain roughly 2g of "protein" per 100g.
One item I don't see that's been mentioned --
Canned tuna fish.
I grew up with tuna melts, tuna salad, tuna noodle casserole, etc.
It's very versatile, doesn't have to be cooked (so very quick to prepare) and stores well so you can stock up when it goes on sale.
Try the Calorie King site - you can search for tons of foods and input serving sizes. Then just apply your local food cost to find out what is the best value for you.
Even though beans might be the cheapest (which is debatable), you have to eat a lot of them to get the same amount of protein that you would from something else. 1/2c black beans with 9g protein might cost up to 20 cents dried, 25 cents canned. 1oz of chicken with 9g protein could cost as little as $2.99/lb /16 or about 20 cents. Really depends on how cheap your chicken is.
I cheated a bit, but here is a link with some details http://lowcarbdiets.about.com/od/whattoeat/a/highproteinfood.htm
If this is right, then tofu may be the cheapest source
If it's meat protein you're after you can't beat game for value and taste. My local game dealer sells pheasant, partridge, duck, venison, rabbit and hare. The flavour from these meats is wonderful (hare fillets are just like fillet of beef) and you have the added vantage of the meat being 100% free range.
I found an exciting solution to my protein problem! The problem with answers so far is that they are too Western-biased, you cannot find supermarkets everywhere but now a very good versatile tip coming. United Nations' suborganization FAO has a goal to increase the protein intake from insects because:
Of the hundreds of insect species reportedly eaten as human food, the most common come from four main insect groups: beetles; ants, bees and wasps; grasshoppers and crickets; and moths and butterflies. As a food source, insects are highly nutritious. Some insects have as much protein as meat and fish. In dried form, insects have often twice the protein of fresh raw meat and fish, but usually not more than dried or grilled meat and fish. Some insects, especially in the larval stage, are also rich in fat and contain important vitamins and minerals. (Source)
Guess how excited I am: frugal, low-cost and reusable solution! Good after sport -- and it is positive to cut some greenhouse gases. Now the challenge is to find out how to get them, editable species, their cooking and their preservation. More about cooking here.