Cost on battery replacement on a iPhone XR?

If you go to Settings>Battery>Battery Health>Maximum Capacity. What is the reading there? If it is above 80%, Apple will not replace the battery. It is is not, then contact Apple Support. You can find cost information on this link. Apple Repair and Repair Status Check - Official Apple Support

You can contact Apple Support through the website or you can download the Apple Support App (Download) and they can guide you to resources for battery replacements and other repairs in your region through an Apple Authorised Service Provider, in addition to prices for the repair.