The auditor general is being asked to investigate the cost of a terminal building at Inverness Airport.
The SNP said the terminal will cost at least £35m
The Scottish National Party claimed that the private finance initiative (PFI) project will cost about £35m.
Transport spokesman Kenny MacAskill said it would have cost £9.6m to build the terminal without PFI.
He called for action by the Scottish Executive, which said it was awaiting the outcome of a review of the structure of the deal.
Inverness Airport is operated by the state-run Highlands and Islands Airports Limited (HIAL).
Under the terms of the PFI deal, it leases the terminal building from its owner.
"This PFI rip off is the Skye Bridge mark II," said Mr MacAskill.
"A building that would only have cost £9.6m to build by paying up front has already cost us nearly £5m after only four years and we still
have 21 years of payments to make.
"At very minimum, without even an increase in passenger numbers, the building
will cost over £35m."
Calling for an investigation by the auditor general, he said public money was being wasted on the project.
"We need to know why this was chosen and who benefits, because it's not the city of Inverness or the people of the Highlands," he said.
"Although it was a Tory administration that got us into this mess, it's a Lib/Lab administration that is now in charge and has been for years.
"It's time for them to act and resolve this scandalous waste of public money."
A Scottish Executive spokesperson said it was not party to the deal which was struck between HIAL and Noble PFI five years ago.
"HIAL and Noble PFI are reviewing the structure of the deal to ensure that it continues to be effective in delivering the intended benefits for the operation and development of the terminal and airport.
"The executive fully supports the review and awaits the company's recommendation.
"We anticipate that the company will write to Transport Minister Nicol Stephen in the near future with its recommendation. The minister will then consider the available options," said the spokesperson.