The newest issue of the Wrestling Observer Newsletter goes over the latest WWE quarterly report. In Dave Meltzer's analysis, he came to an interesting conclusion from the available data:
WWE PPVs do 10% less than they "should" if they take place the night after a UFC PPV, and "even show some weakness" if there's UFC PPV 8 days before. This most noticeably affected Survivor Series '09 and Wrestlemania 26. Wrestlemania did 20% less than "the wrestling side" of the company did predicted that it would be, and 12% less than it was budgeted to do at the end of last year.
Meltzer also believes that Manny Pacquiao vs Joshua Clottey's success in the same month (and thus being on the same cable bill) could have also affected Wrestlemania, as it was a big enough event to have crossover appeal, and could have affected the more casual audience.