I know Silk soy milk is the most expensive, that is always in the refrigerated section...in the healthier food section I have found cheaper soy milk along with the rice and almond milk in the "boxier" containers. I doubt that helps you much, tho. I prefer the texture of almond milk over soy as I found soy milk too thick (and rice milk too watery).

Have you tried watering the soy milk down a bit, or does that just gross you out? You can also buy unsweetened soy milk, which hardly has any sugar...I did that for a long time. Not sure any of this helped, but I am impressed by your dedication, and I wish you luck.