Estimated costs of the Crossrail 2 project have risen by nearly £7bn, according to new Transport for London (TfL) spending plans.
The 30% increase, of £6.6bn, has been due to the inclusion of costs for new trains and surface works, figures show.
It means the north-south rail scheme is now expected to cost £20bn for the shorter Metro route and £27.5bn for the longer regional option.
A decision is expected in early 2015 between the Metro and regional routes.
Crossrail 2 would run from Cheshunt in Hertfordshire to Epsom in Surrey, passing through central London via stations at Tottenham Court Road, Victoria, Chelsea and Clapham Junction.
Construction could begin in 2017 if proposals are approved.
Consultants Mott MacDonald estimated in 2012 the entire scheme would cost £20.9bn, but the increase in costs has been shown in new independent report by consultancy PwC.
It suggests that money from fares could be used to meet the cost increase, but funding may also come from extending the business rate supplement brought in for Crossrail 1 and the mayor's Community Infrastructure Levy (CIL) - a tax on London land owners and developers used to bring in money for big infrastructure projects.
Other options included in the document include a possible council tax rise, similar to that used to help cover the cost of the 2012 Olympics, and further payments from landowners and property developments near the new line.
These measures combined could provide over half of the scheme's cost, reducing the burden on the UK taxpayer overall, PwC said.
Val Shawcross, transport spokesperson for the London Assembly Labour Group, said it "makes sense" for Londoners to contribute to the scheme's cost.
"[But] it is only right that a significant portion of the funding comes from central government and that the cost of Crossrail 2 is not unfairly shouldered by London taxpayers."
A preferred route for Crossrail 2 scheme was announced in October by mayor Boris Johnson, who said the project was vital to support the capital's growth.
The scheme would complement the £14.8bn east-west Crossrail scheme, which is currently being built and scheduled to be operational by 2018.
Supporters say Crossrail 2 would slash journey times across London, with a journey from Kingston, in south-west London, to Tottenham Court Road being completed in 29 minutes - 17 minutes less than today.
But concern has been raised about the location of a new station in Chelsea along the proposed Crossrail 2 route.
Residents of the Cremorne Estate on the King's Road argue demolition of housing may be needed if it is chosen as the location of the Chelsea West station.
The current proposed station location is further east, near the fire station on the King's Road, and received the majority of support in this summer's consultation.
The consultation document said other options will be considered during 2015.