How did the Super bowl XLV affect the economy of the United States?
There is a huge amount of controversy over the impact of sporting events (and sports teams in general) on economies. You can see a discussion of the issue (about last year's Super Bowl) in the dailyfinance.com link below.
Since you are talking about the economy of the whole United States (as opposed to one particular place) it is hard to argue that the Super Bowl has much of an impact. The Super Bowl might affect the economy of the Dallas area since it was held there. However, it should not affect the economy of the country as a whole. Yes, a lot of money was spent on the halftime show and the advertisements and everything, but that money would likely have been spent in some way in the United States if the Super Bowl had not been played. People who spent to go to the game would have spent that same money doing something else and so the economy of the US as a whole would likely not have been impacted.