This post is not sponsored! I just really love "Bob's Red Mill" protein powder. It's the cheapest protein powder that I've found, and I like that it's simple (the only ingredient is soy protein). There's no sweetener, so it's low in calories.
The cheapest source of this protein powder that I know of is iherb.com. A 14-oz bag, which contains 20 servings, is $7.99. That works out to be $0.40/serving. For protein powder, that is really good!
My favorite thing to make with protein powder is a banana cacao milkshake.