I think at auction the Nukla cover would go for about 5-6k. I'd personally be willing to offer 4k for it. It'd go great with my Nukla collection! Based on the past sales for Nukla items I think it would be around here. The first cover sold for $1,400 in 2010. Of course, Ditko wasn't the artist. It was done by Giordano and Sal Trapani. Cover #2 and the complete interior pages sold for $840. It found the owner of the alternate cover for #3 and he wanted $9k for it. Out of line? Not sure, but definitely out of my price range.
I've been more of the book side of collecting and not the art though so my insight might be way off here!