I think he cost her 4 points. Without the Comey letter she would have won by 6 points, even assuming the race closed towards the end, and won by 333 electoral votes, about the same number as Obama in 2012.

Of course, the race never would have been that close if not for Comey's press conference in July. And he absolutely would have held that press conference, Bill Clinton's meeting with Loretta Lynch not withstanding.

I don't even think that there ever was a legitimate basis for an FBI investigation. But Congress demanded one and Comey complied.