Barcelona’s rail tunnel connecting the existing high-speed rail station, Sants, with city center, has just been completed. The tunnel’s total length is 5.8 km. As for cost:
The tunnel has cost over €179·3m to build, including extensive measures to protect historic buildings such as Gaudí’s Sagrada Familia from any settlement.
I believe this sets a new modern-day record for low construction costs – about $40 million per km – certainly in cases of inner-urban construction. It balances out the city’s Line 9 boondoggle, which has run so many times over budget it’s now a full $180 million per km.