Joe Certa's cover art for issue #31 is going for $850 at comicartfans.com. The issue was published in 1975, and includes the story "The Doom of Hellgi Kolnisson." I'm just as confused by that title as you are.
While it's hardly my favorite art from the Gold Key line, it's a one-of-a-kind piece. Also: the art is goofy as hell. There's not a living room in America that wouldn't be improved by adding some framed art depicting a wizard zapping a vampire for the amusement of a (very bored looking) viking. I haven't read this issue, but can assume the viking is Barnabas' distant cousin, Leif Collins. (Note: that image to your right is something I made for for the CHS Tumblr feed last year.)
Trivia: Joe Certa is the co-creator of the DC Comics character Martian Manhunter.
If you want to pursue this item, you can find it HERE.