Graham Matthews, of quantity surveyor EC Harris, said the current estimate of £2.37 billion to deliver the massive scheme in east London could rise tenfold.
Matthews also admitted that it was still 'early days' in terms of design and there was no real way of estimating how much the transformation of the area would cost after the Games had finished.
Speaking to the AJ's sister website www.qsweek.com
at an Olympic Briefing yesterday, he said: 'If you were just building a stadium for 80,000 people you could build it to a price you could understand.
'But when it has to be cut down to 25,000 in legacy mode, we don't have the technical or design answers yet'.
Matthews also cast doubt on the Olympic Delivery Authority's (ODA's) prediction that it will appoint the all-important delivery partner role by September.
'I think [ODA chief executive] David Higgins' timetable to appoint a delivery partner is very optimistic.' Matthews added.
He also described the eight-month delay caused by retendering the role as 'disappointing, but a good idea.'