Well, if it's only for "iron evidence and worst possible crimes" there shouldn't be enough of them to provide lots of money, it'll be just one mine in the country and the prisoners should be able to just cover the legal fees for sending them there. That place will also draw in the worst sadistic-leaning guards making normal prisons safer from them. In an ideal world I'm all for trying to find more civilized solutions (just locking them in solitary even if that means we need to support them on our money), but the reason we need those solutions is exactly because the world is too far from ideal. Just death, even painful death, is too simple of a punishment for someone who is ok with gang-raping a child.