Sorry, £4 billion. That reminds me of all the local papers here putting a £6.9m price tag on Rugby airport, when even £6.9bn was conservative.
Even so, how would a two runway airport in the Midlands only cost a little bit more (according to the White Paper) than a single extra runway at Stansted with no other extra infrastructure? Even if the terminal expands / a new one is built, I believe the original structure was about £120m. With inflation, that might be £1/2bn now? There won't be any new rail route and presumably no new control tower etc...
Or how can they build a whole 6 runway airport at Denver for $5bn - even that was something like 4x over budget. I just don't see how the price of tarmac can be that high. Land in the South East may be pricey, but that part of Essex is not exactly Mayfair.