I'm no civil engineer but I imagine that building an all-seater stadium for over 90,000 with a substantive roof is a logistical nightmare. The comparisons with the Nou Camp are somewhat misleading because Spanish grounds are generally built with much steeper stands. On that basis you are not comparing like for like and it is hard for me to imagine a larger Wembley working. In any event given the shambolic way the FA mismanaged the whole project I shudder to think what that would have ended up costing - and let us not forget that ultimately it is the fans who end up paying for this incompetence not the 'suits' at the FA.
In terms of the allocation there has historically always been a relatively small allocation to the finalists' fans (the 'football family' anyone?) well before the advent of corporate sponsorship - so to blame this on the inexorable march of global capitalism is wide of the mark.
I had been to the 'new' Wembley once prior to the semi and I feel that whilst no-one could describe it as good value for money, it is a truly impressive piece of architecture and the standard of finish in certain parts of the stadium is truly breathtaking.