And that doesn't actually cost that much any more - especially since long term spoil disposal is not an issue since almost all of it will go straight back into the hole afterwards. And at lower depths (since the tunnel is unlikely to be a continuous 100m below ground level) the amount of earth to be moved shrinks rather drastically.
A surface railway is sometimes below and sometimes above grade, depending on the local topography. The design usually aims to balance the cuttings and embankments as far as possible, so the spoil from the cuttings roughly balances the amount needed for the embankments.
If it has to be in tunnel then by definition it can't go above grade, so where the ground goes from low to high, to stay underground in the low part it will be that much deeper underground in the high part so the amount of spoil to be removed and put back will be more than the amount to be removed (only) on a surface railway.
The entire length will also require concrete lining segments to be brought in, and after filling in the trench there will be spoil left over equivalent to the volume of the tunnel plus the lining. As there are no embankments to use this up, it will have to be disposed of off-route. All this will add up to maybe a dozen HGV movements in and out of the site for each metre of tunnel.
For this and other reasons, a cut and cover tunnel will involve far more disruption to the locals during construction than a normal surface railway would. There may be less noise and disruption after completion, but evidence from HS1 indicates that it's reasonably straightforward to mitigate the noise impacts of operational high speed trains.