I would have to say that it is worth about $600 - $650 under the best conditions. I'm sure you paid more than that because I also have an IdeaPad Y470 but with a Core i5-2410m, 4GB of RAM and a 3 year warranty (21 - 22 months left).
The issue is that your laptop is no longer under warranty. And even if you did purchase an extended warranty, it may not be transferable to the 2nd owner. It is still a good laptop for games so I don't think you need to worry about potential buyers looking for a bargain. The i7-2630QM is still a good performer despite the release of the newer Ivy Bridge CPUs, and if I am not mistaken the nVidia GT 635m is the equivalent (or slightly faster) to the GT 550m.
I think I value my Y470 at about $600 assuming the warranty is transferable, otherwise I would probably value it at $550.