Councillors are due to look at a business plan for the Grade II* listed complex in October so they were bemused when they saw a figure for £148 million slipped into a report.
Housing officers said the figure was based on "approximate estimates" made 18 to 24 months ago before developers Urban Splash came on board.
Officers say assumptions were made about the scheme and its costs and it was an error to include such a "speculative estimate" in the report.
Joanne Roney, executive director for neighbourhoods, said: "The estimated cost given in the report is an out-of-date figure which predates the selection of Urban Splash as the developer. It is an error.
"The council is currently awaiting the Urban Splash business plan that will clarify the overall cost of the proposed project and until then any figures quoted are purely speculative."
Previous debate has centred on a figure of about £40 million for refurbishment.
Lib Dems blamed Labour and said they had handled the issue badly.
Lib Dem leader Paul Scriven said: "Despite the controversial nature of the scheme they have refused to be up-front and honest about the project and its cost to the taxpayer. It's clear to me that there are funding problems with the project and the plans just don't stack up.
"It is now time for Labour to admit it was a mistake to list the complex in the first place and take all the necessary steps to move towards demolition."
He said it looked increasingly likely Sheffield taxpayers would end up paying for another "Labour failed project".
Conservatives said the "monumental monstrosity" should not have been listed.
Coun Anne Smith said: "There are arguments about the amount of money that will be spent on it and the whole thing is a mess. There is too much money at stake for any further experiments with this blot on Sheffield's landscape.
"Apply to get it de-listed then knock it down and use it as hardcore for motorways."