One of the drivers of cost has been the high speed part. It meant the route needed to be straight which meant more tunnelling to avoid certain areas. Lower speed track would have allowed them to route around problem areas.
The vast majority of the tunnelling for HS2 has been either to get in and out of dense urban areas without too much demolition, or to reduce the political cost of driving a building site through Tory heartlands (Chilterns AONB).