High school preparation for US student looking at Oxbridge/Imperial

He would probably come back to the US.

I think your statement is probably true apples to apples – say, Oxbridge v. HYP. But since any top-ranked US private school is out for us financially, I wonder if that’s still accurate for, say, Oxbridge v. Ohio State. I really don’t have a clue. Or as @PurpleTitan says, if that would still be true for grad school.