> But that's completely ignoring any geological differences of course, so not very realistic. I could imagine that there are several other features of a tunnel

> But that's completely ignoring any geological differences of course, so not very realistic. I could imagine that there are several other features of a tunnel that might affect its price considerably: cross-section dimension, number of pipes, technique of construction, security installations, ventilation, the type of use (rail, automotive, both), ... For example, the Fehmarnbelttunnel/Femern Bælt-tunnelen you mentioned will consist of four pipes for both, rail and automotive, and is not drilled but built out of prefabricated modules (42m x 9m x 217m) made of concrete placed in a trench on the seabed and then covered with stones and sand.[1] [1] Source: https://de.wikipedia.org/wiki/Fehmarnbelttunnel https://de.wikipedia.org/wiki/Fehmarnbelttunnel (in German)