Arguments made against LHR R3 on the grounds of cost are entirely coherent. Expansion there makes sense operationally, but that is not the whole story. Value for money must be considered where a substantial contribution from the national exchequer will be required to make it happen.
(1) There is a long term massive boost to the UK economy to be had (in terms of new overseas links with expanding up-and-coming markets) which is good for trade, exports, and inward investment, and this will more than compensate for any public sector involvement;
(2) most of the cost would be borne by the private sector;
(3) some of the public sector costs (i.e. surface access infrastructure) will be required even if there is no third rwy, with a third rwy, LHR Ltd would probably contribute, perhaps for M25 and A4 tunnels.
Perhaps not so coherent after all.
To be fair, Edinburgh is a more important city than Manchester
Is it really? airport pax numbers would suggest otherwise.