It's quite possible they could reduce the cost if they found a way to pipe the excavated rock into surrounding regions of less dense rock, through pipes drilled

It's quite possible they could reduce the cost if they found a way to pipe the excavated rock into surrounding regions of less dense rock, through pipes drilled at intervals along the tunnel. Meaning the excavated rock never needs to go up to the surface, it can just be digested and fed as a slurry into neighbouring areas.