I recently sold a Late 2013 27 inch, i7 3.5GHz, 32GB RAM, GTX780M 4GB, 1TB Fusion Drive for the equivalent of $1000 USD. I was satisfied as it was about what I expected to get.
I would say $700 to $800 would be about what you could realistically expect for your specs, depending on the condition of the machine and the storage which you didn't mention. I'd go a little higher for pure SSD.