The iPhone 8 and 8 Plus' most obvious design change is the switch to a glossy, glass back. There was a chance this glass would be priced at or around the same price as a cracked screen under AppleCare+, but that's apparently not the case: AppleInsider confirmed today that a broken back won't qualify for a $29 screen replacement. It'll instead be subject to an "other damage" fee of $99 (plus tax).
Apple gives users two screen replacements and two miscellaneous damage incidents at the $29 and $99 price respectively. After that point, the damage price jumps to $349 for the iPhone 8, and $399 for the iPhone 8 Plus. So basically, don't crack your phone's glass or else you're going to be dishing out lots of cash. AppleCare+ costs $129 for the iPhone 8 and $149 for the iPhone 8 Plus.
We don't have pricing for the iPhone X’s repairs yet. We know Apple will charge $199 for its warranty program, but the company hasn't said how much it'll charge for a screen replacement or broken glass. We can probably assume a new screen will cost more than $29, as the screen is already supposed to be somewhat limited in supply. Cherish the original screen or pay up, I guess.