There's an assumption in your question. You asked, "How much of a premium do creators' personal copies fetch?".
The way that question is worded makes it sound like you're taking it for granted that they fetch some sort of premium. Some of them might, but your first question should have been, "Do creators' personal copies fetch a premium?".
My answer would have been, "It depends." It depends on the book, it depends on the condition of the book, it depends on what creator owned the book, and it depends on who is buying the book. I wouldn't pay a premium for any creator's personal copy of a comic book—prior ownership of a comic simply doesn't matter to me—but in many cases a collector who admires a particular creator would likely pay a premium for that creator's personal copy of a comic.
Prior ownership of a comic clearly does matter to some people. The Larson copies prove that: some board members have stated they they are willing to pay more for those copies because of the backstory even though those comics often have heavy foxing, rust on the staples, or both.