Walmart sells a brand called Sun Soy that my son used to drink for between $1.60 and $2.00/half gallon (they keep changing the price). It is NOT organic soy milk, that’s why it’s cheaper than some of the others.
They don’t generally have coupons for it, I emailed the company and that’s what they told me.
This is the cheapest brand I could find anywhere. Silk is reasonable but it’s usually about $2.50/half gallon or more.