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. Recipes Of these, soy milk is the most labor intensive.
Nut milks are straight forward and fast.
Grain milks, such as rice or oat milk are tricky. 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. For almost all home varieties of grain milk, it will taste more strongly of rice, oats, soybeans, etc, than the commercial alternative. This creates soy milk as a by product of making tofu.