The inquiry under Lord Hardie was announced in 2014 and its evidence sessions were completed more than two-and-a-half years ago, but it has given no indication as to when the findings will be completed.
Further funding of £500,000 has been included in the 2021-22 Scottish Budget published yesterday, up from £200,000 in 2020-21.
It is thought the extra money will increase the cost of the inquiry to £11.8m because the Scottish Government revealed last September it had reached £11.3m.
The inquiry is investigating why the cost of the tram project increased from an £375m to around £1 billion and was completed years late in 2014 after an acrimonious dispute between its builders and a city council company leading the project.
A spokesperson for Transport Scotland said: “The increase [in the 2021-22 budget] is because the prior year’s budget was informed by an early estimate the year before, which was based on the inquiry concluding that year.
“The inquiry has not yet completed and Lord Hardie continues to write the report.
“The funding covers the cost of this work, including a small support team and relevant advisers.”
The Scottish Conservatives said people would be angry at the “huge” cost of the inquiry.
Iain Whyte, leader of the opposition Conservative group leader on the City of Edinburgh Council, said: “It’s now almost seven years since the tram Inquiry was announced and it’s already taken longer than the original delayed construction.
"The public will be astonished to find the ongoing costs of report writing are now two and half times the original estimate.
"The fact that the estimate was based on the report writing concluding some time ago brings the further delay into sharp focus.
“While we all want a thorough and accurate report into what went wrong, it is now imperative the report is issued as soon as possible.
"Lessons learned are more useful the sooner they are available and the huge and unexpected cost of this inquiry is just adding to taxpayer anger over the mismanagement of the original project.”
A spokesperson for the inquiry said: “There is no update as to when the report will be published.”