My Aug 2018 X says $5,000 to upgrade from AP to FSD.
My Dec 2017 S said the same thing when I upgraded, but Tesla refunded $1,000 as I was told the upgrade would be $4,000 when I bought the car and then upgraded at a later date. I suspect the same would happen with the X as the purchase agreement said the same $4,000 price for upgrading to FSD after the purchase of the vehicle.