Well, the cost to put on the show, for one. It's not a cheap production.
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Official video of the performance:
[nomedia="http://www.youtube.com/watch?v=hUYzQaCCt2o&feature=relmfu"]YouTube - Roger Waters + David Gilmour: Comfortably Numb, Live, O2 Arena 2011[/nomedia]
and "Outside The Wall" with Roger, Dave, and Nick:
[nomedia="http://www.youtube.com/watch?v=uZqS7LAyup4"]YouTube - Roger Waters, David Gilmour, Nick Mason and The Bleeding Heart Band: Outside The Wall, Live O2 Arena[/nomedia]
I must admit, seeing them together again gave me chills.
If this does happen this will be a major departure of any feeling Waters has had previously. He has stated time and again he wants to continue to move forward and "not look back" on his music. It is interesting that he is now doing a Wall tour that obviously has so much personal meaning for him.
Oh and regarding gouging of his fans, I doubt he has a lot to do with this. The prices of these shows are the going rate for these shows now and have a lot to do with the promoter's, venue costs, etc. Waters is in the top 100 richest citizens of the UK with a net value of over 105M pounds from what I have read. Gilmour and Mason are very well off as well. This might be one show I would be willing to fork out that kind of dough to see.
Overall, though, anyone who is a fan of the "Floyd" has to be happy at the thawing of relations here.