Mr Buswell, who is also the state's treasurer, says there is an $864 million "hole" in a major part of the project, a link to Perth Airport.
"I asked the PTA (Public Transport Authority) to look at a whole range of transport options ... but one we've looked at in particular ... is the link to the airport," he told Fairfax Radio on Friday.
"We've spent a lot of time in government working out the preferred route to the airport. We haven't made any announcements about it but we do have a preferred route."
Mr Buswell said he knew "damn well" that the airport rail plan, which would involve a line being built down Tonkin Highway and sinking it "effectively below the water table", was very expensive.
Labor estimates the 10.5km airport line will cost $731.5 million, not including rail cars, but Mr Buswell expects it will have a $1.595 billion price tag.
Mr Buswell said treasury would cost the party's proposal if it submitted detailed plans.
Treasury can cost opposition policies after the pre-election financial projection statements are released.
"The Labor party can submit their costings to the treasury - completely independent of the government - next Friday and treasury has a requirement under the caretaker conventions to provide that cost back to the Labor party within three days.
"They should do that."
"If they're using government resources to do that, release them," he said.
"We'd love to see them.
"Mr Buswell makes things up. He will have included in his costings the most expensive options available."