I did a similar project when I was working toward my MBA - tried to determine the economic impact of the Carolina Panthers. Nice perk was getting to visit the stadium, meet with Jerry Richardson, go on the field, etc. Eventually presented to people from the Panthers and various government and business leaders from the city. As you discovered, it was crazy hard to come up with some sort of accurate financial figure. It was basically a bunch of guessing and extrapolating. Like it seems you did, we really talked up the "intangible" impact, like how having professional sports teams makes a city a "big league" city and can help draw businesses and what-not. It was half B.S., but probably wasn't much different in that regard to what the pros do.