Lots of good data (and a map!) on this subject can be found at https://transitcosts.com, much of it compiled by https://twitter.com/alon_levy

Also, here in NYC, it isn't just tunneling that's expensive [0], we pay dearly for most of our infrastructure. As this article points out, the elephant in the room is labor costs.

[0] https://twitter.com/marketurbanism/status/118704548054032384...