I think the number would be huge. I grew up and lived in Illinois for most of my life but fled 11 years ago. Everyone I knew there that could leave has left. Those that stayed did so because of work or they have kids in school.
Illinois was a great place once, but between the illegal wetbacks, high taxes,high crime, and the legendary Firearms Owners Identification Card (FOID Gun Card), there's just no reason to be there anymore.
I know I would have left had I lived there. Yes, the jobs and kids, along with financial constraints were the reasons I was referring to by stating if they could. I have a friend that lives up there and he remains only because his wife does not want to move away from the kids, who are all grown and have kids of their own.