Article doesn't mention costs, but this one (two miles long) probably cost him b/w 1 & 2 billion, and that's rather a low-end estimate.
No direction given of the tunnel from their headquarters … waiting for the next earthquake around LA, and how that tunnel will hold up. Kek.
LA is in a basin (amplifying seismic shake), but probably making it easier to tunnel because of sedimentary environment. Also Musk's tunnel is said to be narrower than usual.
Still a lot of faults around there – would have to check accurately where HQ & SpaceX facilities are to see if tunnel crosses any of those.