I think to reach that 750 figure it would need to be a proven prelim, and not just a sketch from a convention reproduced FROM the prelim.
Bernie had not done sketches at shows for YEARS (before his passing). Those are actual prelims.
I agree, however that it might not be worth $750
Wanted
CD Carrie Portfolio 719
Dark Tower S/N LE's 171 or 203
ANY Stephen King S/N LE #171 or 719
A Storm of Swords #218 or 346
Ancillary Justice #455
American Gods (+ SC Reader copy) #624
Michael Whelan original art
DT VII: Michael Whelan Remarque