Using the example of the proposed Donald Trump Mexico-USA Border Wall; various figures for the estimated cost have ranged considerably.
Mr Trump claims the total cost of the wall will be $10bn (£7.5bn) to $12bn. But estimates from fact checkers and engineers seem to be universally higher. Senate Majority Leader Mitch McConnell estimated it will cost between $12-15bn, as he addressed reporters at a Republican conference in Philadelphia.
A study by the Washington Post estimated the cost of the president's wall would be closer to $25bn
Another example might be the proposed High Speed Rail 2 in the UK. The Department for Transport initially estimated the cost to be £30 billion; in June 2013 the projected cost rose by £10 billion to £42.6 billion. The Institute of Economic Affairs estimates that the final cost will be over £80 billion.