expired Bonsoy Long-Life Soy Milk 1 Litre $3.50 @ Coles (Nation-Wide)
Numerous past deals, most recently here.
Cheapest I've seen these are when they are in a box combined with AMEX statement credits. These are close to be the cheapest in a main-stream supermarket where it is accessible to everyone, all payment options are available with fly-buy points.
Happy stocking up!
Q: But I don't like soy milk?
A: Why are you here?
Q: blah blah blah Bonsoy is poisonous blah blah blah
A: See previous discussion and FAQ's
Q: blah blah blah Vitasoy is cheaper blah blah blah
A: Cool, we are all free to buy whatever brand we like