Personally, I think the underlying book is basically still the same value as unsigned. The Frazetta signature itself adds a minimum of $1,500(?) as some have suggested. Then, the "specific combo" of book and signatures which are most desirable to a particular collector, for whatever reason (is it a key? is the signature placement awesome? etc.), adds any additional premium above a simple calculation of GPA + $1,500.
Over time, I assume the "flat estimate" of $1,500 for the signature will continue to increase.
I don't think the $1,500 thing really works.