It was originally estimated that a probe into the controversial transport scheme would cost no more than £1million.
But despite not a single witness being called, and Lord Hardie not due to finish his report until 2018 at the earliest, a Scottish Government spokesman has put the total budget to the end of 2016-17 as £6.5 million.
He said: “The total budget for the tram inquiry is £6.5million. The budget for 2017-18 has not been determined but we would expect that is would conclude in that year.”
Some politicians fear the inquiry could cost more than the £10.2 million Chilcot Inquiry into Iraq.
Scottish Conservative MSP Miles Briggs said: “This is clearly in danger of becoming more expensive than the farcical Chilcot inquiry.” Labour’s Edinburgh South MP Ian Murray said: “The tram project has cost enough without throwing another £10million into it.”
The Edinburgh tram project cost £776million - more than double its original estimate.