The cost of 14 miles of tunnels (and crossing the Thames and some smaller rivers, crossrail is under $2.5b. The entire project including half a dozen new stations entirely underground and a ton of electrification is $20b. That includes threading the line through a very congested underground area - tunnels within 12 inch of existing tunnels etc. The cost of the 2 mile branch of the pair of tunnels under the Thames was about $350m, cheaper per mile than the main tunnels.

The jubilee line extension crossed the Thames 3 times and cost $5b for the entire project including new underground stations and 6 miles of new tunnels.