The independent audits of expense claims by four senators at the centre of a spending scandal cost taxpayers more than $520,000.

The Senate’s Internal Economy committee said in a statement Friday that the contract for audit firm Deloitte to review Sen. Pamela Wallin’s living and travel expenses cost $390,058.00, more than double what she was ordered to repay in ineligible claims.

In the end, Wallin was ordered to repay $138,969 in invalid expense claims.

The statement said the final value of the contract was amended “in order to provide for additional work required, but not originally anticipated,” including a review of Wallin’s travel claims from Jan. 1 to Mar. 31 2009; a review of living allowance claims from Jan. 1 2009 to Sept. 30, 2012; and a review of “electronic information.”

“We know this is a significant cost,” Sen. Gerald Comeau, former chair of the committee, said in the statement. “However, once the audit was ordered, we had to allow it to be fully concluded in order to get a fair and consistent reading of the issues involved.”

Meanwhile, the contract for reviews of the travel and living expenses of Senators Mike Duffy and Patrick Brazeau, and now-retired senator Mac Harb, came in at $138,784.

“This contract was also amended in order to provide for additional work required, but not originally anticipated,” the statement read.

“The cost of conducting these audits has to be viewed in context of the larger issue of public accountability and trust,” said Sen. George Furey, former deputy chair of the committee.

Deloitte declined to comment on the audits Friday, saying in a statement to CTV News that it “takes very seriously the confidentiality requirements of the work we do for our clients. Our policies and our code of professional conduct prohibit us from discussing any information about clients or the work that we do for them.”

The firm also said that its audits are conducted “in strict accordance with the highest professional standards.”

After the audits were completed, Harb repaid some $230,000 in disallowed travel and other expense claims and resigned from the Senate over the summer. Brazeau was ordered to repay about $48,000, and the Senate began garnishing his salary after he missed a repayment deadline in June.

Duffy’s expenses were repaid with the help of $90,000 from Nigel Wright, Prime Minister Stephen Harper’s then-chief of staff.

All of the audits have been referred to the RCMP. The Mounties are also investigating the Wright-Duffy deal.

Wallin, Duffy and Brazeau are now facing suspension from the Senate over the scandal. After much debate and procedural wrangling this week, senators will likely vote on the suspensions motion on Tuesday.