Update 23/02/2016: HS2 Ltd CEO Simon Kirby, the highest-paid civil servant in the UK, has confirmed that the official figure for the predicted cost of HS2 is £55.7 billion.
Update 10/09/2015: As predicted, the sum has increased. The best guess is now £138bn.
Sir David Higgins, Chief Executive of HS2 Ltd, has stated that costs could rise and the overall cost could reach £50bn. Higgins was responsible for the 300% overspend on the 2012 London Olympics, which eventually cost £9.6 billion and saw negligible economic benefits, particularly outside of London.
Reference: The Times Online