Costco is a great place to get cheap nuts, as are local ethnic or specialist foodstores, and stores that sell them in bulk. The bulk section is always cheaper than buying nuts pre-packaged. If you're looking to make the cheapest milk possible, I'd recommend making sunflower seed milk. These seeds are full of vitamin E, selenium and magnesium. They're totally underrated because they're a bit bland in flavour, but are really good for you and are way cheaper than any other nut or seed milk!