Giving evidence to the Environmental Audit Committee on Heathrow's third runway, the transport secretary discussed warnings from TfL that making such improvements could cost up to £20bn.
The Airports Commission has estimated surface infrastructure changes will cost £5bn, but TfL said the figure underestimated the cost and required changes would add up to a total of £15m-£20m.
"I think it's ludicrous to be honest," Grayling said.
"I do not see where £15bn can come from if you look at what we're actually seeking to deliver around Heathrow Airport."
He said it looked to him as if "somebody has taken every possible transport improvement in the whole of metropolitan London and thrown it into the mix, and probably funding a large chunk of Crossrail 2 out of it as well".
He noted that changes to the M4 were happening already as part of the government's road improvement process, while M25 changes will be funded by Heathrow Airport itself and rail improvements will be partly funded by the airport and partly by the government. "
These are long planned projects which would happen regardless of the airport expansion," he said.
He added that the launch of Crossrail, HS2 providing better connectivity and TfL's plans for the Piccadilly line amounted to "a huge step forward".
"I'm baffled as to where TfL manages to get a £15bn figure from because I don't know what you'd spend the money on."
Grayling also said he expected Heathrow to plump for the so-called runway ramp option to cross the M25, rather than building a tunnel as it will be "operationally easier as well as less expensive".