iPhone X cellular antenna how much price

Apple doesn't sell parts to individuals.

see https://support.apple.com/en-us/HT201415

Apple won't sell one to you. You will need to do an out-of-warranty replacement of the phone, click on the Contact Support link at the top right of this page.

The only place you can change just the antenna is at a non-apple third party repair shop and they will need to give you a price.

Apple will only replace the whole phone as has been explained.