According to rumors, the new prices (at the highest end) are as follows:
iPhone 6.1 - $700
2018 iPhone X - $900
2018 iPhone X Plus - $1000
If the new iPhone X is indeed $900 (so the Plus can start at $1000), then last year's X should technically be $800 brand new (although they would obviously just discontinue it so they don't cannibalize sales of the 6.1). Current X owners are already going to be devalued automatically at $200 so I think there is a really good chance to pick up a used 2017 iPhone X for $600.
iPhone 6.1 - $700
2018 iPhone X - $900
2018 iPhone X Plus - $1000
If the new iPhone X is indeed $900 (so the Plus can start at $1000), then last year's X should technically be $800 brand new (although they would obviously just discontinue it so they don't cannibalize sales of the 6.1). Current X owners are already going to be devalued automatically at $200 so I think there is a really good chance to pick up a used 2017 iPhone X for $600.