Soy is a cheap food, yet national brands of soy milk cost around $3.50
per half gallon, which is often more than real milk.  The dairy lobby
has secured subsidies on real milk, but have they also convinced
Congress or the USDA to add a surcharge to the cost of soy milk?  In
other words, is there a special tax on making soy milk or other
products (soy cheese, soy butter, rice milk, etc) that compete
directly with dairy products?

Just a free comment:
I expect that the price of soy milk is related to that of cow's milk,
regardless of the cost of production.  Soy milk appeals to persons
with special interests or needs (vegetarians, people with milk
allergies) and can apparently achieve the same or a higher price.  If
it could be sold much more cheaply, I assume that soy milk marketers
have somehow determined that the great majority of people would still
buy cow's milk, so that they would just be reducing their income
without significantly increasing sales.

Yes, it is possible that demand for soymilk is somewhat inelastic up
to the price of cow's milk.  Perhaps the similarity in price between
dairy and non-dairy substitutes is due to manufacturers' desire that
they should be considered equivalent and interchangeable.