The commission says a second runway at Gatwick would cost £2 billion more than the bid suggests.
Two separate plans to expand Heathrow are predicted to cost £3-4 billion more, the BBC reported.
The commission has been tasked with determining where a new runway should be built to meet rising capacity demand in the southeast.
It is considering three options – a second runway at Gatwick, a third runway at Heathrow, or an extension to one of the existing Heathrow runways.
The commission has released its latest analysis of the three shortlisted plans at the start of a three-month public consultation period.
But commission chairman Sir Howard Davies (pictured) said today that no decision had yet been made.
“We have not yet taken a view on which proposal strikes the most effective balance between the assessment criteria,” he said.
“It is important first that we provide an opportunity for this evidence to be examined, challenged and improved. This consultation gives everyone with an interest in the issue of airport expansion that opportunity.”
However, the report suggests that all three bids have underestimated the costs involved.
Landing charges to airlines would also need to increase in order to pay for the expansion.
Gatwick’s plan to add a second runway is the quietest and easiest to deliver, according to the commission’s analysis.
But expanding Heathrow is seen as more likely to deliver a bigger boost to the economy, and create more jobs.
The commission is not due to issue his final recommendation to the government until the summer of 2015 – after the general election.
British Air Transport Association chief executive Nathan Stower, responding to the Airports Commission’s consultation on its appraisal of the three shortlisted options, said: “The Airports Commission’s final recommendation must be cost effective, offer value for money, and not rely on today’s passengers paying for infrastructure that won’t be ready until at least the mid-2020s.
“UK airlines will study the appraisal of each option in detail to judge whether the plan is supported by a robust business case with realistic forecasts and a credible funding mechanism.
“I hope our politicians will wait for the results of this consultation and the commission’s final report so that political decisions can be taken with the fullest evidence.”
Sourced from Travel Weekly