The cheapest protein sources are plant-based– lentils, tofu, beans. If you think about it, most traditional diets worldwide are predominantly vegetarian or vegan. While some of the specialty products, like coconut ice cream, are expensive, it's easy to be vegan on a very low budget, especially if you can cook for yourself. There's a blog called plant-based on a budget, where they post recipes that can feed a person for $25 per week, or just over $1/meal.
