They are currently dirt cheap, because Tesla isn't looking to profit (this is a PR stunt and stock pumping excercise for now).

The rides cost $4.20. Haaah, that joke will never get old, Elon :|

And yes, the reason that it impacts the stock price so radically, is that labor is a huge portion of professional driving expense. If you can make every professional driver unemployed by automating their job, you can make a small handful of people very, very rich.