You heard wrong. Expect a 15%-25% decline at minimum, depending on what happened the last few hours before the deadline.
I think that would be unprecedented in JTS history. I looked up the yearly average attendance for every season played in JTS (1975 - 2014, available for free at Cyclones.com) and only once has there been a decrease in attendence of that magnitude. In 1996, we averaged 43,371 attendance followed by 36,487 in 1997 - a decrease of 16%. Really, the 1996 attendance was the outlier though because you have to go more than 3 years either direction from 1996 to find an average attendance greater than 39,000. I know attendance is different than season ticket sales, but I'd bet the correlation between the two is pretty strong and I doubt there's any way to get season ticket purchase numbers outside of the Athletic Dept.
From 1982 - 1986 we lost 24.5% of our average attendance (51,906 in 1982 to 39,162 in 1986).
From 1990 - 1993 we lost 19.7% (44,271 - 35,551).
From 1996 - 1998 we lost 18.8% (43,371 - 35,181).
From 2007 - 2010 we lost 8.2% (49,462 - 45,395).
A decrease of "at least 15% - 25%" would put our average attendance figures squarely into the 1990s realm again. Assuming the UNI and Iowa games are sellouts (assuming 61,000 attendance), then the average attendance of our final four games would have to be about 30,000. The only game on the schedule I don't see us drawing decently to is TCU who will likely destroy us. Otherwise it's KU, Texas and OSU. And our last home game is November 14 - early enough that the weather may still be bearable, and students will have another week of class before leaving for Thanksgiving. Plus SEZ, plus there is an outside shot that this team could be playing for bowl eligibility if we can win those first four (maybe five???) games.