The Director's Cup is probably the best way to keep track of things like this - of course, it puts equal weight into all sports, but it's as good as measure as you'll get across the college sports landscape :
You hit the nail on the head. It doesn't make sense to weight field hockey as heavily as football. How many fans have ever even seen a field hockey game? Or for that matter, even know how many players are on a FH team or how long a game lasts? (I don't think NU even has a FH team, right?)
If you applied any sort of weighting system to the fall sports we'd probly be 40 spots higher. For example, here's a 1-100 ranking I'd suggest for the fall sports: Football = 90, Volley Ball = 40; Soccer = 30; Cross Country = 10; Field Hockey = 10; Water Polo = 10). Such a ranking (which I just made up, eyeballing it, btw) would put us in the 20--maybe even top 15.
I think you may be missing the point--this is a look at how competitive our athletic teams and overall program is. The Director's Cup is an extension of this thinking. Attendance or popularity doesn't enter into the equation, nor should it--just whether or not the athletic teams our school put out to compete were successful.
Just because there aren't 86k sets of eyeballs on each and every game, that shouldn't diminish the success these kids had at Nebraska in their sport. Hell, we've had championship Bowling teams at DoNU, and I doubt very few stopped down to watch them, but it doesn't diminish their significant accomplishment, nor should it, and it contributes to the overall strength of the Athletic Department.