Crossrail is 118km and is going to cost 16bn GBP or about 135m GBP per km (~$209m/km) (if I've done my maths right.)
Though it's somewhat difficult to separate out the 42km of new tunnels (expensive) and a whole bunch of new stations (expensive) including a whole bunch of revamped ones in the centre of London (very expensive).
Though it's somewhat difficult to separate out the 42km of new tunnels (expensive) and a whole bunch of new stations (expensive) including a whole bunch of revamped ones in the centre of London (very expensive).