You might be right Ardy jan, but I always thought that United offered more but he chose Barca because he thought it would be a better fit.
Look at this:
http://www.footballeconomy.com/stats/stats_turnover_10.htm
This is old, but I think if you look over 10 years, it is much more reflective of the two teams spending powers. Arsenal's spending comes largely from player sales, their ability to win in competitions and what that gives you in commercial revenue. If you took that away and Arsenal and United had to go head to head, United would trounce Arsenal. It's not just transfer fees but how much you pay the players and United can offer much more. Arsenal only signed Rosicky for money this summer. They did get Adebayor, Walcott, and Diaby in the winter break but only Walcott was for 5 million which will rise to 12 depending on appearances. There were rumors that Adebayor was a huge signing but I remember Wenger saying in an interview at the time that it wasn't as much as people thought. Vidic was no bargain, he was signed for 7.2 million pounds and Arsenal were after Evra but I suspect couldn't get him because of money. I'm not saying Arsenal are poor and I think Ferguson often makes much better decisions than Wenger when it comes to selecting a team, but in terms of money, United are a giant compared to Arsenal.