Going off of this, this is what I deemed to be the "local population" for each Big Ten school. For most of the classic state schools, I used the total MSA. The only exceptions are below:
(A) I used Middlesex County, NJ for Rutgers given how massive the New York MSA is.
(B) I used Prince George's County, MD for Maryland for a similar reason.
(C) I used the city of Evanston for NU given its extremely selective fan base and unique nature among Big Ten schools as a small private school. Also, I know from living here that Evanston is probably the only place in the Chicago MSA where non-alum fans might default to Northwestern.
Minnesota: 3,712,020
Ohio State: 2,180,271
Maryland: 967,201
Rutgers: 863,623
Michigan State: 473,177
Michigan: 365,536
Nebraska: 344,387
Illinois: 235,608
Purdue: 226,564
Iowa: 180,088
Indiana: 160,874
Penn State: 157,795
Northwestern: 78,110
FWIW, we would have about 515,000 additional people in instate counties immediately adjacent to the Champaign-Urbana MSA, bringing our total to about 750,000 potential fans to draw from that could feasibly zip over to a game and drive home that day (if they don't enjoy as many tailgate beers as I like to! Lol...).
Either way, it seems certain that our better attendance numbers rely on quite a lot of people from Springfield, Peoria, Chicagoland, the rest of Central Illinois and (to a lesser extent) the Quad Cities, Metro East and Rockford areas. I stand by my assertion that a Friday night game probably hurts our attendance significantly more than the conference average (our historic futility being included as a reason here).