The BBC has spent around £5 million investigating the Jimmy Savile sex scandal and its aftermath.
Its annual report and accounts reveal the Pollard Review, which looked at why the BBC dropped a Newsnight investigation into Savile, accounted for almost half of that.
It cost £2.4 million which included £101,000 to cover the "legal and related costs" of Helen Boaden who was heavily criticised in the report.
The then head of news was among the senior executives who were criticised for failing to act while the BBC was plunged into chaos by the scandal.
Figures show the cost of the review and subsequent investigations into respect at work and the BBC's culture and practices while Savile worked there have cost £4.9 million excluding tax and VAT up to the end of March.
The third investigation, which will also examine the case of recently jailed Stuart Hall, will be published later this year sending the final bill even higher.
Writing in the report, BBC Trust chairman Lord Patten quoted Charles Dickens to compare the success of the Olympics coverage with the Savile scandal saying "It was the best of times, it was the worst of times".
He said the revelations about the scrapped Newsnight investigation and subsequent departure of director-general George Entwistle were "low points".
He said: "The BBC seriously let down both itself and licence fee payers".
New director-general Tony Hall said he wanted to change the culture at the BBC and called for "greater personal accountability" and a simpler corporation.
In a letter to Lord Patten he said he had been "struck by the complexity of the organisation and inhibiting effect that has on creativity."
He added that he was "personally leading a major piece of work to look at how we can simplify our organisation".
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