There's a lot of tunneling on HS2 - including massive underground stations at Euston and OOC, a new station in Birmingham, and tunnelling in London and Chilterns.
That has to happen whether the trains run at 180mph or 80mph.
LGV Mediterranean has to negotiate the hilly terrain in southern France. Plenty of tunnels and viaducts. €20m/km. Way less than us. The problem is with how we manage construction projects in the UK.
You'd be amazed just how poorly dynamite works for making miles of tunnel, in between up to 12 layers of pre-existing tunnels, through wet central London clay.
That has to happen whether the trains run at 180mph or 80mph.