If the problem is not covered by the one-year limited warranty, i.e. a manufacturing defect, a screen repair (read: screen replacement) will cost you a flat fee of $279. Everything else? You'll pay $549, more than 50% of the original price of the phone. So expect to see a lot of broken and probably never to be repaired iPhone X backs.
You can, of course, anticipate these problems and buy the one off $199 AppleCare+ which will cover up to two incidents with a $29 deductible for screen damage and $99 for other damage.
It does sound like AppleCare+ might be worth the investment if you have slippery fingers but adding another $200 to an already crazy starting price of $999 is a tall ask. Have you pre-ordered the iPhone X, and what do you think?