Current official estimates for the proposed UK railway line High Speed 2 are £42.6 billion for a 192 km line, or around £222 million (€287 million) per km (if my calculations are correct). A 2020 review suggests it could even cost as much as £106 billion (€126 billion), or £552 million (€656 million) per km.
The French LGV Est total cost is around €4 billion for around 300 km, or €13 million per km — a factor 22 less than the initial UK price or a factor 50 less than the 2020 high estimate.
The German Nürnberg-Erfurt high speed railway line total cost is estimated at around €5.1 billion for 190 km, or around €27 million per km — a factor 10 less than the initial UK price or a factor 24 less than the updated price.
The Swedish planned Östlänken line is estimated to cost around 30 billion SEK (€3.2 billion) for 150 km, or around €21 million per km.
The French, German, and Swedish high speed railways are all in a range of 10–30 million €/km. Why is the cost for the UK High Speed 2 a factor 10–50 higher? What makes it so expensive?
The costs for a third runway at Heathrow Airport are estimated to be £18.6 billion. For comparison, the 2003 Polderbaan was €320 million, which is far less even when taking into account inflation. Quoting the Financial Times:
While an additional runway would indeed seem desirable, it would not be worth paying such exorbitant costs. Why such projects are so expensive and so difficult to implement in the UK is, to me, the big puzzle. It cannot just be because southern England is densely populated. So is north Holland, where Schiphol, with far more runways than Heathrow, is located.