From this week's DfT report's Executive Summary:
"The revised analysis set out in this report suggests that, as was the case for the Airports Commission's analysis, the net present values (NPVs) of all three schemes, generated by subtracting the monetised costs from the monetised benefits, are close. The NPVs of the two Heathrow schemes are subject to more uncertainty than the LGW Second Runway scheme. The LHR Northwest Runway delivers the highest NPV at the upper end of the central range, and the LGW Second Runway delivers the highest NPV at the lower end. This is due to the uncertainty surrounding scheme costs and surface access requirements as a result of airport expansion, which are still to be determined. The ranking of the schemes by NPV is dependent on a number of assumptions necessitated by these uncertainties."
Further Review and Sensitivities Report - Airport Capacity in the South East