Could anyone give me an idea on on how much a misprint Magic card could sell for? It's a Graverobber Spider, rarity is uncommon.
As murgatroid says, there isn't a good estimate for any card, but there are a few guidelines. Major misprints are more valued by collectors than little ones, so miscuts where some other card from the sheet is visible are better than off-kilter cuts or shadowed text. More famous or rare cards typically have more demand, because misprint collectors are still collectors, and they still want prominent cards.
From that criteria, I can't give an exact figure for your spider (and especially because I don't know the nature of the misprint), but unfortunately I think I'd have to guess it isn't worth much more than a non-misprinted one.