You are building tunnels, rather than track, hence the price should be that of a tunnel, not plain track.
To make the track tool have a cost that varied would confuse people according to prissi - even if the tool tip was altered.
For the sake of discussion, there could be an expensive "generic" tunnel with no tracks nor road. Once it's built, you can pave it with roads at their normal cost or build a track at its normal cost.
The same has been requested in the past for bridges (and I still would love it)...