As a point of reference for the cost to move up, Penn State moved both of their men's and women's hockey programs up to NCAA D1 in 2012. Terry Pegula, a Penn State alum and now owner of the Buffalo Bills and Sabres, donated $102M to build new facilities, including a 6,000 seat hockey stadium.
It is worth noting that before Penn State moved up, the Big 10 was not a hockey conference. Big 10 hockey started in 2013 with Minnesota, Wisconsin, Michigan, Michigan St, Penn St., and Ohio St., later adding Notre Dame as a hockey only member.
All this to say, if ISU had one or a group of passionate donors to sponsor a new hockey complex, there is a case to be made for being the eighth member of Big 10 hockey.