In the Netherlands I think you might get about €1000 for it,
if you're lucky. It really depends on what SSD's and graphic card(s) are in there etc etc.
iUsed.nl, here in the Netherlands, have Quad core 5,1's for around €700-€800. Keep in mind that they are a business and that their prices will no doubt be higher than on the second hand market.
Think you're realistically looking at €750, I'd be interested to hear what others have to say.