https://www.cnet.com/news/get-an-unlocked-iphone-x-for-700/

As you've probably noticed on the product page, you also have the option of getting the phone for $649.99 -- but that's if you activate it with AT&T, Sprint or Verizon. My advice: Buy it outright, then take it to a carrier with cheaper rates. You'll save in the long run, and you'll have the option of switching carriers anytime you want.

The elephant in this particular room is the iPhone XR, which runs $750 for the unlocked 64GB model. Personally, I think they're both great phones, but there are a few key differences.