The report by the Comptroller and Auditor General also said the Commission was originally given two years to complete its work, at a cost of between €1.9 and €2.5m.
However the Commission, which published its findings in May, took more than ten years to complete its work.
Legal fees contributed to a significant proportion of the costs. The Commission's most recent figures suggested that the final costs of third-party representation could be between €52m and €62m.
The report also estimated that administration costs will amount to €30m, while the commission's legal team will cost almost €16m.
It said some support work was done using counsel when less expensive staff could have been used.
