ESPN OTL did a study on the last six years of college ADs finances (2007-08 to 2012-13). Michigan led the Big Ten during the period with a total of $90,243,483 of profit, averaging $15,040,580.50/per year.
Ohio State was number #1 in the country in 2012-13 with a surplus of $24 million. Michigan was #4 in revenues ($143.5 million), #3 in expenses ($131 million), & #7 in surplus ($12.2 million) in 2012-13. I was a little surprised at how much Wisconsin made in 2012-13, ranking #2 in the country in revenue and expenses, but I guess this may be dependent on how many home football games a school has and how schools maintain their financial books.
Big Ten specific analysis from Big Ten Blog: http://espn.go.com/blog/ncfnation/post/_/id/95196/b1g-numbers-revenues-and-expenses