Here's another estimate of the cost. As this was in October 2008 no doubt it has risen since then.
Eric Pickles uncovers cost of 2011 Census
As National Statistician I have been asked to reply to your recent question asking what the estimated total cost to the public purse is of the 2011 Census, and how much has been spent to date on trials prior to that Census. (230808)
On the basis of present planning assumptions, the cost of the 2011 Census in England and Wales over the period 2005-2016 is currently estimated to be around £487 million.
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