This is how Elon Musk tends to operate. The initial models are higher priced to help pay for the tooling, design and manufacturing process. That's why the Model S and X are expensive, to help pay for the Model 3 I am sure the PowerWall is similar.18k seems like gouging to me and it's probably a ploy to rip the wealthy off at the start (cause they'd like to be first with their shiny new green technology) then the prices will drop significantly once the initial hype is over.
I still remember talking to a sales rep at the Farmfest show thingy only about 4 or 5 years ago and getting quoted 16k for a 5kw system that cost us 8k a few years later. It's even less now!
The power wall will probably be the same - over priced to begin with then cheap as once competition comes in and they need more sales.