The iPhone 8 and iPhone 8 Plus have a glossy back panel instead of aluminium like the iPhone 7, iPhone 6s or iPhone 6. Replacement will be much more expensive than replacing the screen.
According to AppleInsider, the cost of replacing the back of the iPhone 8 and iPhone 8 Plus is $99, much more expensive than the cost of $29 when replacing the screen. The cost of replacing the back is included in the cost of repairing the "other damage" of the iPhone.
Apple will allow users to replace the screen twice for $29, and replace two other "damages" (including backpacks) for $99. If broken for the third time, the replacement cost will be up to $349 for the iPhone 8, and $399 for the iPhone 8 Plus. So, if you own the iPhone 8, users should "raise as eggs, inspirations like flowers," otherwise they will have to spend a lot of money to replace components.
We do not know how much the repair cost of iPhone X is. Apple currently sells the iPhone insurance package for $199, but Apple has not announced the cost of replacing the screen and the iPhone X's back. But the price tag instead of the flagship version of Apple will cost more than $29.