So far I'm up almost $200. I'm sure it would be possible to make a cheap one for around $150.
$17 minimum for a system.
$40 minimum for a screen.
$40 minimum for a Wiikey Fusion. (optional)
$50 minimum for batteries.
Then add various other parts like adapters, chargers, small components, etc...
These are VERY rough estimates. It greatly depends on how you build your system.