Musk's plans outline an initial route between Los Angeles and San Francisco with an estimated travel time of 30 minutes. His analysis suggests it could be built for under $6 billion, paying for itself in 20 years with a ticket price of $20 in today's prices.
Of course, Musk's plans are no more than that. Musk isn't planning to develop any of the technology he describes in his white paper; nor does he have any plans to tackle the difficult political and logistical problems of actually getting the system built. So while it's a nice idea, there's no way to know if it's practical without a lot more work.