The public inquiry into the Renewable Heat Incentive (RHI) has cost around £4.6m so far, the Belfast Telegraph understands.
The inquiry's 111 days of oral hearings drew to a close yesterday, bringing to an end months of revelations which offered an eye-opening insight into Northern Ireland politics.
Over 60 witnesses appeared before inquiry chairman Sir Patrick Coghlin and panel member Dame Una O'Brien, and more than a million pages of evidence were presented.
While the inquiry's website estimates that it will rack up around £6m in direct costs alone, we understand that, as of September 30, costs were in the region of £4.6m.
It is also understood that a couple of days have been set aside in December for legal representatives of the core participants to make representations to the chairman and panel.
The date on which the inquiry will report back is not yet known, but it is believed that it could be well into the new year.
The first person to give evidence was Alison Clydesdale, who appeared on November 28 2017. The last witness was the former head of the Northern Ireland Civil Service, Sir Malcolm McKibbin, who rounded off the process yesterday.
As the oral process drew to a close, Sir Patrick told Sir Malcolm: "It's just nice to hear someone with a little bit of confidence and a lot of ability addressing the panel for once."
The investigation into the botched green energy scheme has laid bare the inner workings of the Civil Service and Stormont's main parties, as well as revealing the key role of special advisers in the running of the devolved government.
High-profile political figures to appear included DUP leader Arlene Foster, former DUP Enterprise Minister Jonathan Bell, and former Sinn Fein Finance Minister Mairtin O Muilleoir.
Among the headline-making claims were those of Mr Bell, who said he was the victim of a "massive smear campaign" at the hands of the DUP.
He denied bullying accusations made by his former special adviser Timothy Cairns that he had attempted to break his finger and swung a punch at him.
Meanwhile, Mr Cairns claimed in a witness statement that Mr Bell drunkenly sang Deep Blue Something's hit Breakfast At Tiffany's in New York after being asked to leave a bar.
During her appearance, Arlene Foster, who was in charge of the early RHI scheme while Enterprise Minister, said she was not aware it was £200m more expensive than an alternative scheme that she rejected.
In her witness statement, Mrs Foster said she was "unaware" of whether any business or individual who benefited from the RHI scheme was a DUP donor.
DUP chief executive Timothy Johnston denied he held a position of seniority over other special advisers in the party when an overspend in the RHI scheme came to light. He also said he "had no involvement in the decision-making processes on the introduction of cost controls in 2015" in his written evidence.
Dr Andrew McCormick, the top civil servant at Deti when the botched green energy scheme was set up and collapsed, revealed that Stormont meetings about RHI weren't recorded for fear they would end up in the papers.
Dr McCormick also said that officials "got used to" special advisers ruling the roost at Stormont.
In his evidence, he said that DUP adviser Andrew Crawford knew about the abuse of the RHI scheme and had told him "we could fill our boots" as he believed the money was coming from the Treasury instead of the Stormont budget.
In his evidence, Mr Crawford, who was Mrs Foster's special adviser when she was Enterprise Minister, denied that he was involved in any efforts to delay the imposition of cost controls to the scheme in 2015.
But he apologised for emailing an internal document about RHI, which had not yet been released to the public, to his cousin Richard Crawford, a poultry farmer.
During the inquiry, it also emerged that former Finance Minister Mairtin O Muilleoir had sought the consent of Gerry Adams' confidant Ted Howell before signing off a key RHI document.
Head of Northern Ireland's Civil Service David Sterling said there is "a strong case for a review" of the role of Stormont's special advisers before devolved government is restored.