I have a friend who makes his own and it is dirt cheap if you can get the beans cheap enough. If you have a coop or some place you can buy them in bulk then it may well be 8p a liter (it's too late at night for me to convert currency AND volume in my head...) but if you're buying them in little sealed bags from Tesco it's probably going to be more.
Frankly if I lived where I could buy So Good soy milk then I'd be getting that, expense be darned
That stuff is awesome and I can't find a soymilk in the US that's remotely comparable so I really don't drink as much as I should. And now I'm craving Alpro pudding and custard. Sniff, I miss Tesco