Posted By:
P SpaethLemme give you my typical answer: it depends. 
It probably does depend to some extent on the terms of any warranty PSA might have, is it based on cost or is it based on value. But in terms of general principles of fairness, you are really only out $100, right? Damages often are assessed as the difference between the actual world and the "but for" world. In the actual world you have a worthless card. "But for" the misgrading you would not have had a $1000 card; you would have the $100 because you wouldn't have bought the card in the first place. So your damages are $100 plus interest.