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.