iPhone XR Replacement glass back

Here are the published Apple prices: iPhone Repair - Official Apple Support

Based on that list, it appears this will probably be $99, as an “other” repair. Assuming no other damage, etc.

Bringing your device into the folks at Apple, or to your preferred hardware service provider, will get you a much better estimate.

The rear glass can not be repaired and the device must be replaced.

If they purchased AppleCare + it will be $99. If not it will be $399 USD.