Here is one of the latest lists: http://www.wealthx.com/wp-content/uploads/2014/10/Top20BillionaireInstitutions.pdf
Several caveats here: 1) these publications routinely understate/undercount Michigan graduates; that said, this one finally bumps the oft misquoted total of 7 such graduates to a more plausible figure; 2) yes, this is undergraduate only and not total; 3) billionaires are tail events and don’t say much about the median/average student, but at least to the extent that it is a beauty contest, Michigan is in the running and good publicity is better than no or bad publicity; 4) there are a few guys like Brad Keywell and Tony Fadell (Madonna?) who are in the “on-deck” circle and may join the club soon…thus boosting the count into the top 10 or so…