We talking paid attendance or butts in seats?
The announced, paid attendance will probably be about 80-85k. That is what paid attendance was during the second half of the 2012 season when the fans gave up on Dooley, and I expect this will be similar.
Butts in seats could be as low as 60k. I think you'd be hard pressed to ever have attendance much below that because 1) there are some fans who will come no matter what and 2) people who don't come to many games and families, especially families with young kids, like to come to games like this because the environment isn't as rowdy and the partying/drinking/tailgating scene is nowhere as intense.
I know people who love football but don't necessarily come to games because they "don't like big crowds" (IMO, that's one of the reasons you go to the game, but whatever floats your boat). They like coming to games like this, so that keeps attendance from being something horrendously bad like 40k.