There seem to be several people in there who don't have 250 posts - someone with 170 posts was giving me grief the other day.
Anyway - a good rule of thumb with cases is that for top line cases up to 1 year old, 75% is a reasonable starting point IF it's perfect (no scratches), boxed, complete and you haven't modded it at all.
Then knock off 5% if it's externally scratched, another 5% if it's not boxed, another 5% if it's been modded etc.
Generally, they're worth about half the original purchase price