now that is a good question. I am not a huge Ditko or Strange fan (outside of Defenders) so I can't really answer but I am sure some experts will chime in.
I can definitely see a collector valuing something high and possibly higher than something published if it truly strikes a chord with them but that would be the exception to the rule and an outlier scenario. So I can definitely see unpublished work being valuable, especially when it is a complete story … I still would not see it as MORE valuable than published. The problem is likely to find that one collector who values the art AND is willing to (over)pay for it. It does happen but is rare from what I have seen. Thinking of myself, the perfect storm would be a Sal Buscema Hulk story that was done but not published. If completely drawn and done at the level of a published work, I would certainly be interested. That said, if the same money could get me a published story vs an unpublished, I think I would opt for published every time.