The $77 billion price tag was the most recent year-of-expenditure estimate (which kept going up,in no small part because the year of expenditures kept getting later) for the whole originally planned system (well, the SF to LA part, not including the Sacramento and San Diego connections that were to be done afterwards.)
The small segment Newsome refers to isn't anywhere close to $77B.
Operating & maintenance costs would also be critical to determining whether this actually makes any sense.
How much do the roadways cost?
I mean, the Prius isn't an all-terrain vehicle. You need nice roads to run, and freeways to go somewhere fast. Without them you can't go anywhere in your Prius.
So how much do the roads you need to exist to drive around in those Prius cost?