Sure, if the loss-rate were high, your math works. Mind you, the SnowGeese only have to complete 25 flights each, and they break even. After that, every flight reduces the cost. Plus, the million buys the support gear, too - Additional units are surely less then 500K each, and can be supported by that same gear. Then, there's the fact that the SnowGeese can do things the Sherpa could never do, such as service multiple locations on one run, and go places the Sherpa can never go, such as many, many miles *past* the drop point, or being launched from the ground when there are no aircraft about. How far you think the forward logistics camp is going to huck a Sherpa-equipped crate without air support? If they have a big enough trebuchet, they might be able to deliver to the base's outer sentries. Aircraft time is pretty heavily allocated. With this system, a couple clerks can load up a drone, drive down the main drag of the base in a borrowed truck to launch, then go get lunch, securely confident that their delivery is on its way. Likewise, I'd like to see a Sherpa try to do that nifty leaflet drop...
The Sherpa has its place, but don't be fooled into thinking that the SnowGoose is an expensive boondoggle. It's actually very inexpensive for what it does.
Oh, and about vulnerability - a helicopter has a body of up to 10X the volume of the SnowGoose, and is far, far louder. To shoot a SnowGoose down, you'd need to 1) notice it, and 2) hit a much smaller target. Not so easy as it sounds. Even if you *do* hit it, well... Its parachute is already deployed!