$1500, at least. And the higher grade the book, or the more rare the book is in grade, the more base value I would put on it. So, you may be able to get a 9.8 Blue Vampi #1 for $100, but I think that is a $1500 -1800 book.
GPA on a Vampi #1 in 8.5 in 2014 was about $450. RMA got $2800 for his 8.5 Frazetta SS copy.
Right around that time, I got an offer of $3500 for my 9.0 with a GPA around $700 for blue.
So an absolute beater .5 Thunda or Weird Fantasy book might fetch $1500, but a 5.5 or an 8.0 would see a much greater spread on the "value" of the Frazetta book, because it is a better vehicle for the sig.