Store bought oat, rice, soy, almond, potato and coconut milk is expensive due to shipping costs, a thin market, and the tetra-pack they come in. Rice milk is also expensive due to the import quotas placed on rice to subsidized a miniscule number of US rice farmers at a massive cost to consumers. It is so hard to boycott when so many farmers and agribusiness concerns have been lost to the dark side. In any case, making your own milk at home will be less expensive than storebought milk of any sort, from cows or grains.
Of these, soy milk is the most labor intensive. Rice and oat milk are straight forward and fast. Almond milk is inbetween. If anyone has a recipe for potato milk, send me email email@example.com. Cost per gallon is anywhere from 50 cents to $1 a gallon. Multiply your current wage per hour times 1/6 to factor in the cost of your time. You probabably already have the equipment, so that doesn't factor into the cost. The electic and gas costs thought probably are less than a half dollar, especially if you use a pressure cooker or soak your grains.
Just about any juicer or blender will do. However, here are some manufacturers that specifically mention rice or soy milk.
The tetra packs are recyclable. Buying in bulk sometimes cuts the cost. however, the prices range from $4 to $16 a gallon, which is still double the inflated price supported cow milk prices of about $2.
Mail order is theoretically possible, but UPS shipping might make it too expensive.