Whey Protien is so expensive

So I'm going to ramble on here about it.
I read about this "cheap" diet where you drink protein waters instead of eating every other day to loose weight. On the days where you eat you do a bit of exercise and try and eat healthy.
But holy Jesus that stuff doesn't come cheap.
So 900g works out at about ten days worth of dieting, that will cost me twenty pound and that's with it on offer! That means I'll be feeding myself for two pound a day. Which, ok, its not that bad until you break down what I currently pay for food. I feed me and my boyfriend for less then that!
The only ones that are cheaper are full of carbohydrates so they won't make me loose weight.
Someone buy me some?