$300 is for a WHOLE cue. Not just the shaft.
Most people already own a butt though so that would in many of their opinions be $140 wasted for nothing when they already could or even prefer to use the butt they have. The choice of a $160 shaft, or $300 to get the shaft along with a butt you don't need, is an easy one for many/most.
If these $169 shafts make it to market though, the guy selling the $300 whole carbon fiber cue will probably be the first one to go ahead and offer just his shafts for the same price though. He already values them for more or less around that amount, he just won't currently let you buy them separately but I would have to think that would change as soon as their is a shaft competitor in the same price category.