Still sticking with my estimate, $499.
This thing will be likely Zen based, RX 490 equivalent ($300+ GPU card. In comparison PS4's have had a closer to $200 GPU and they launched at $400), and 12GB of GDDR5. To target native 4K, I just can't see it for less than $499.
e=mc^2
Gaming on: PS4 Pro, Switch, SNES Mini, Wii U, PC (i5-7400, GTX 1060)