I was on the website this morning looking into tickets for various games.
I would estimate that at least 60% (maybe more) of the non-student seats are still available for the September games, which probably gives us a good indication of where season ticket sales are.
The prime sections along the west sideline and the South end zone are mostly sold out, but plenty of tickets are still available everywhere else in the stadium (even for Iowa/homecoming). The prime sections on the east sideline are still maybe 40% available. Huge sections of the north end zone are barely touched.
Based on what I saw, I would guess that maybe 18,000-22,000 non-student season tickets have been sold so far.