Spring Game attendance over the last decade-ish:
Year - Attendance-Change
2003 - 33,419 ... 1,999
2004 - 61,417 ... 27,998
2005 - 63,416 ... 1,999
2006 - 57,415 ... (6,001)
2007 - 54,288 ... (3,127)
2008 - 80,149 ... 25,861
2009 - 77,670 ... (2,479)
2010 - 77,000 ... (670)
2011 - 66,784 ... (10,216)
2012 - DNP ... --
2013 - 60,174 ... (6,610)*
2014 - 61,772 ... 1,598
* (2013 attendance change based on 2011 Spring Game)
The two biggest jumps in attendance were after regime changes, in 2004 to see Bill Callahan's "new offense" and in 2008 to see Bo Pelini take over. I remember thinking how odd the crowd was in 2004, so many people. And I remember the huge ovation when the offense lined up in the I-formation, then motioned into a more pro spread set. Pretty funny.
Offhand, I'd expect about 75,000 for this year's game. Depends on how harsh the winter is, how nice April seems like it's going to be, and probably on the buzz around the team from Spring Camp. If reports are "meh" coming out of camp, it'll probably be more like 60,000.