From this independent cost benefit analysis for the 2002 Commonwealth Games:

"The public investment in the Games and the associated regeneration infrastructure and activity has increased from £640m to over £670m, at 2002 prices, of which some £570m has been or will be in East Manchester. The net additional public sector investment of the Games venue construction and operation are some £320m at today's prices, of which some £120m was contributed by Sport England, with a further £225m of regeneration spend associated with the pre-Games and parallel activities. The Games have, in addition, brought forward a number of major transport schemes (with a total cost of nearly £800m), and we estimate the additional public investment in transport infrastructure to be £125m."

If the Olympics were to be on that scale of cost, fair enough. But those figures are small change compared to what's happening in London. A Money Pit.