Madrid built a subway tunnel (Metrosur) for about $60M per km (about 41 km length in total) in 2003. That's about $97M per mile. Tho, the tunnel is actually out

Madrid built a subway tunnel (Metrosur) for about $60M per km (about 41 km length in total) in 2003. That's about $97M per mile. Tho, the tunnel is actually outside of the actual metropolitan area, Berlin calculated some planned subway extensions (U1, U8 lines) at $120M to $280M per mile (2017), tho, those are short extensions of existing tunnels of about 1 km each and only estimations not actual costs.