I did a quick search and the market pricing appears to be more in the $6000-$7000 range at the moment.

What is the hybrid case for the ammo that they talk about?

The SIG Fury ammo comes in a three-piece case designed to handle the higher than standard pressure released when fired. I don't know the specifics, but the rifle and ammo were developed to provide a lower-weight platform that still delivers a punch.