They would probably take most of it if not all if you never paid any taxes on that money. If you didn't, I'm sure they can come up with all sorts of fines and penalties that add up to whatever amount of money you got saved up. If you did pay taxes and can prove it then they won't bother you nor take your money.

Sometime ago i saw on the news the story of this undocumented man from Guatemala, I believe, who got detained trying to leave the country with 60k cash in his pocket and the government took it away from him because he'd never paid any taxes+fines+penalties (trying to leave the country with such amount without reporting it) and deported him. I don't know the details though but I'm sure IRS can be a bitch if you owe them money!