IMS is so big, that you can put 100,000 in there and the place looks empty. I know because I attended several of the F1 races there. Before the 2005 Michelin-gate fiasco, the average attendance was 120,000 ... and the place looked stone empty. The race was cancelled after 2008 even though it was the highest attended event on the F1 calendar, mostly because Tony George wasn't making any money despite the high attendance (compared to all other F1 events) and Bernie didn't like the way "it looked on TV", read: too many empty seats. Perception is reality, and if people watch on TV and see empty seats, it detracts from the event, no matter how many people are actually there.
I have said it before and I will say it again, we are at a crossroads for ALL spectator sports in America. Technology is so good (HDTV, specialized coverage from the cable/satellite providers, DVRs, internet and social media) that is tough to beat what you get at home. Plus, the ancillary costs are absolutely userous when it comes to attending an event. I honestly don't believe too many people have problems with tickets prices per se, but when you factor in outrageous prices for parking, beer, soda, souvenirs and hot dogs ... it makes it real easy to stay home and watch your favorite sport in HD and chat with your friends on Facebook or make snarky tweets.