The blaze at the disused recycling facility in the Deptford area in May took three weeks to fully extinguish.
The Local Democracy Reporting Service (LDRS), said a report priced up the “unrecoverable costs” of moving waste, and subsequent air monitoring.
An investigation by the fire service found it was started deliberately, and police said enquiries were continuing.
The site was abandoned when waste management firm Alex Smiles went into administration in May 2015, leaving creditors out of pocket by £7.5m.
Les Clark, chief operations officer at Sunderland City Council, said: “The council and others had been in negotiations about the future of the site, and the risks it posed, before the fire broke out.
“The council continues to work with partners with a view to bringing the site back into use.
“However… several million pounds will be needed for this.”