$20 plus operating costs. That is, the $20 is just the additional cost to pay off the capital cost over $20 years, and doesn't include any of the amount necessary to pay the operating costs.
Airline tickets would be pretty cheap if all they were paying was the 20-year amortized capital cost of the airplanes + airports, too.
$20 is the unsubsidized 20-year-amortized cost of building the thing, there would be additional costs to run it, not the least of which being insurance.
Time is money too. The drive from SF to LA is about 5.5 hours (per Google Maps). 5.5 hours of stress and frustration, compared to 30-40 minutes of hyperloop...
Just as an anecdote.. We had some friends from Orange County come visit us in SF this past weekend.
* OC -> SF -- Left at 3pm, took 9.5hrs to drive
* SF -> OC -- Left at 9pm, took 7 hours to drive
There is random night construction as well as detours and traffic to worry about on I-5. Half an hour would be a no-brainer, even if it only went so far as downtown LA.
It's kind of cool that Elon Musk is competing with himself here -- Hyperloop vs. a 3rd generation Tesla (which should be even cheaper than $45 one-way per car, including depreciation and electricity.)
The real problem, even once you add self-driving cars, is I-5's limited capacity.