I think part of the price tag is exclusivity. There's simply no way a $700 helmet will sell as many units as a $150 one. You'd also have less sales to spread out your research and design to.
You also have to account for the midrange lineup slowly picking up features that used to be exclusive to the high end helmets.
I thought this was amusing:
Arai's exceptional handmade craftsmanship, made to a higher standard of protection, comfort, and quality that's difficult to put a "price" on.
We can't explain why it costs more.
Joking aside, the nice thing about Shoei is that you know it's actually made in Japan. Getting that last 20% is costly.