My iPhone XR degraded battery and possible symptoms it may cause

bad battery will mean a shorter battery time and may also mean sudden drops in %

cost to replace battery should be listed here

iPhone Battery Repair & Replacement - Apple Support

also

have you read About the battery and performance of iPhone 11 and later - Apple Support

and maybe iPhone Battery and Performance - Apple Support

and Batteries - Maximizing Performance - Apple

people report apple does not change the batteries until they are lower than 80% in health

When the battery health maximum capacity percentage goes down close to 83-80%, it can have unpredictable behavior and it should be replaced as soon as possible (after creating a full backup of your device's content).

Get an estimate here (select your country):--> Choose Your Country or Region - Apple Support

Regards

Giulio