Not sure if it has been posted before. Visible currently have Pre-owned, very good, iPhone X 64GB for $423. It has been out of stock for awhile, but I just saw it back in stock.
If you do the math, it will costs you around $423 + $50 (party pay 2 months) + tax = $473 + tax out of pocket. If you take the $200 Prepaid Mastercard into consideration, the net cost for the iPhone X will only be $273 + tax (or less if you are using any cash back site).
It seems to be a decent price even if it is a pre-owned iPhone X.
Edit: The promotion terms stated that you must port in a number to receive the $200 Prepaid Mastercard