I revised my rule-of-thumb after seeing the costs for the NHS IT systems [1]. The Olympics going so far over budget really didn't surprise me.
> Originally expected to cost £2.3 billion (bn) over three years, in June 2006 the total cost was estimated by the National Audit Office to be £12.4bn over 10 years ... Officials involved in the programme have been quoted in the media estimating the final cost to be as high as £20bn, indicating a cost overrun of 440% to 770%.
> Originally expected to cost £2.3 billion (bn) over three years, in June 2006 the total cost was estimated by the National Audit Office to be £12.4bn over 10 years ... Officials involved in the programme have been quoted in the media estimating the final cost to be as high as £20bn, indicating a cost overrun of 440% to 770%.
[1] http://en.wikipedia.org/wiki/NHS_Connecting_for_Health#Costs