I think Utd fans will pay ã5 less than the equivalent Arsenal fan for the same ticket - the away supporters generally don't get to sit in the centre of the north or south stand where the tickets are ã49. We have to keep them away from the prawn sandwiches...
I didn't realise Cat A matches were only the minority. That makes the ticket prices a little less outrageous, but the season tickets are still steep at twice the price!
IIRC Queensland Road was originally a mix of affordable housing, offices, commercial space, and facilities for Arsenal. The aim was that the affordable housing would break even, and the offices and shops would make enough money to cover the new facilities (mostly gym and parking). But Islington scaled the development down, so I'm not sure how the financial are going to work out now.