How much to repair cracked back of an iPhone 8
How much does it cost to repair cracked back of my iphone8
iPhone 8, iOS 13
Question marked as
The back glass is fused on your iPhone and cannot be repaired or replaced. The entire phone will have to be replaced as either one of the two replacements at lower cost if you have a valid AppleCare plan on your phone or more expensively as an out of warranty replacement at about 60% of the cost of the phone when new.
This link explains the pricing --> iPhone Repair - Official Apple Support
See the section for other repair costs.