That's the annoying thing, computer hardware depreciates in value quickly.
4 years ago I spent £700 on an Acer gaming laptop, which is now £250 on ebay. The specification of said laptop is Intel Core 2 Duo 2.0ghz, 250gb HDD, 4GB RAM, 17" screen, NVidia GeForce 9500M GS 512mb, Windows Vista.
Of course, I wouldn't use that as a reference for pricing yours because A) yours is newer B) yours is of a superior make (as a proud owner of a Lenovo, I would value it higher than an Acer) C) yours is a better spec. But it's an idea of how value depreciates.