It's not seven times longer. The Austrian line is 130km with 50km of tunnels.

            Length  Tunnels  Bridges   Stations   Cost
    Koralm  130km   ~50km    100       12         €6b
       HS2  230km   ~75km    100+      4          €74b+

Obviously this does not give any indication of the complexity of each project. Tunnelling and building railway through a metropolis I would imagine is quite challenging.

As far as I can see, the 6bn is _just_ for the big 30km tunnel? Presumably the rest of it cost more.

Still seems insanely more expensive in the UK. I understand they have a higher cost to carry because their project is indeed more complex, but that's like a almost 13x more expensive variant, while not even being two times the length.

HS2 is five sets of twin bore tunnels, so there is more "tunnel per tunnel"

Sounds like you might want to build the whole HS2 underground to save money.

Yeah because it would be extremely expensive and we don't need it.