The inquiry into the Edinburgh trams project has cost more than £3.7 million so far, according to figures from Transport Scotland.
Lord Hardie is leading the probe into the scheme which went significantly over budget and was delivered years later than originally planned.
The inquiry was set up in 2014 by the then first minister Alex Salmond.
It is looking at the project, examining its governance, management and other areas to try to find out why it was delayed and why it ‘’cost considerably more than originally budgeted for and delivered significantly less than was projected through reductions in scope’’, according to its remit.
It is not known when Lord Hardie will report his findings.
Figures published by Transport Scotland in response to a BBC freedom of information request revealed a total of £3,708,000 has been spent on the inquiry already.
A breakdown of the costings shows £1,822,000 has been spent on staffing, £502,000 on accommodation costs, £716,000 on legal fees, £540,000 on IT costs and £128,000 on miscellaneous costs.
The trams began operating at the end of May 2014 after six years of disruption and a long-running dispute between Edinburgh City Council and contractors.
The eventual cost of £776 million was more than double the sum earmarked for the project by the previous Labour-led administration.
The Scottish Government has agreed to continue to fund the costs of the inquiry until its completion.
A spokesman said: “We have been clear from the outset that the inquiry should be efficient and cost-effective, however we know that major infrastructure projects do generate large quantities of documentation and we understand the inquiry team has already gathered over six million documents.
“Lord Hardie and his team is firmly committed to discerning the facts in a robust and thorough manner in order to ensure the final report is fair, balanced and offers clear recommendations for the planning and construction of future projects.”
A spokeswoman for the Edinburgh Tram Inquiry said: “The Scottish Ministers have been clear from the start that the Inquiry should be timely and cost effective.
“To date substantial progress has been made including identifying, retrieving and reviewing more than six million documents and the ongoing process of taking extensive statements from witnesses of interest.
“This work is essential to ensure that the Inquiry can proceed in an efficient manner in order to produce a comprehensive, fair and balanced report that delivers best value for the public purse.”