The government slipped out the cost in a written answer before the House of Commons broke up for the summer, last week.
The continuing difficulties at the Inland Revenue have brought more support for Computer Weekly's Making The Tax System Work campaign.
Labour MP Frank Field said, "It is difficult to defend the tax credit system as a targeted benefit when 1 in every 6 is wrongly allocated. Major software problems and technical difficulties have compounded the problem for many families on low incomes.
"Every MP knows the horrors this chaotically run benefit has brought to all too many families paid the wrong sum at the wrong time, and sometimes in the wrong year.
"Greater transparency in HM Revenue would allow for more effective compensation and accountability in the future," he said.
The IT Faculty of the Institute of Chartered Accountants in England & Wales added its backing, "We support Computer Weekly's campaign. Ongoing media coverage about the problems with tax credits and self assessment highlights the fact that if the data being processed is incorrect, the output will also be incorrect, however good the actual processing.
"The negative stories suggest that more consultation by HM Revenue and Customs on the likely complexities of proposed systems could be of great benefit to all concerned."