The branch to Manchester, as currently proposed, is just unbelievably expensive. Perhaps the focus should be on making it less so? Would using some of the existing route from Crewe to Stockport be such a bad thing? Is a connection to the Airport really worth the cost of the major deep level works involved? Perhaps just a grade-separated junction to get the line into new platforms alongside Piccadilly? Would eg. 300m platforms for Stockport and Piccadilly do the job? A more progressive upgrade from Liverpool eastwards, perhaps as a higher priority given that the existing routes are so obviously slow and both lead to the very congested routes into Manchester? Wouldn't four tracking of some the Chat Moss route be a cheaper and quicker option?
Those that visit France will see here that the TGV planners generally have gone to some lengths either to upgrade the existing corridors into major cities (eg. Paris, Lyon, Bordeaux, Marseilles) or build bypasses in open countryside to avoid the costs of new lines in major cities (eg. Avignon, Montpellier, Lyon-Satolas, the Interconnection route via CDG and the first stages of the TGV Rhin-Rhone). The only exception is the wholly new line at the edge of Lille's city centre for the LGV-Nord branch to the UK via Lille Europe, built at the tail end of the Mitterand period.
Contrary to popular belief in the UK (and here!), the TGV network here has also faced budgetary constraints right from the start and SNCF has often been quite cautious in its proposed scopes, making significant use of existing lines (eg. to access Dijon, Switzerland, Grenoble, the Atlantique region and, until very recently, Bordeaux). The long-running failure to agree a wholly new route from Marseilles along the Cote d'Azur to Nice, obviously a very difficult terrain to construct in, is the best example of this (with the current plan being to upgrade most of the existing alignment), as is the plan for a further TGV to Clermont-Ferrand and possible extension to Lyon to provide further Paris-Lyon capacity (on hold). Whether the Mont d'Ambin tunnel can be built to budget, however, remains to be seen....!