I'm not sure I trust any figures quoted by the media. They depend on the base assumptions used and the media never provide that context preferring to go to 'sensational' headlines.
Such variance in figures (£2.2bn to £18.4bn with the independent view of £5.7bn) makes me question what growth assumptions they each use. A Times article suggested 20% growth used by TfL was an "optimism" bias. So what would the figure be for a "pessimism" bias or even the "base" case?