One store sold milk splash (a related product) for $3.50, and I got it on sale for half price, so that was good. They stopped selling it as soon as I found it, of course, and it was the only physical store that I've ever found that sold anything like it. The closest thing I've since found is Johnny Moo on Amazon, which sells 3-packs for $18-20, which works out to over $6 each. That's a massive markup. These things should be no different than Mio, which is now a product category unto itself with lots of competitors, and you can always find one or another on sale for comfortably less than $3/each. There doesn't seem to be any reason why the drops you add to milk (or Soylent) should cost any more than drops that you add to water.
Hence my desire for Soylent to make their own drops. If you haven't tried them with your 2.0, by the way, I highly recommend it. The strawberry flavor makes the Soylent taste just like Strawberry Quik, which somehow feels wrong, since Soylent is a meal and Strawberry Quik, back when I used to drink it as a kid...wasn't.