It will also depend on what the water actually damaged. I've come across three Retina 15" models with liquid damage, and successfully resuscitated two of them. One would not POST; all I did was open it up, unplug the battery, wait a minute, plug it in, and it came up and runs fine. One had the trackpad go bad, but worked otherwise. I ended up tearing it apart (literally, since they glue the batteries to a plate under the trackpad) and replacing it with the trackpad from the third Retina (which had logic board damage and was not recoverable).
If you can determine the damage and how fixable it actually is, you can try to get more for it. If you know the logic board and storage are good you can leverage those. The flash storage is replaceable so anyone with your model type Retina with 256GB may be ripe for an upgrade.