Screen Mirroring itself is not a chargeable feature built into the iPad when using Apple's native AirPlay feature to an Apple TV or an AirPlay 2-compatible smart TV.
The built-in AirPlay screen mirroring functionality on an iPad is free to use, provided you have the compatible Apple TV or smart TV, which is a separate purchase.
Third-Party Apps and Alternative Methods
The confusion about charges often arises from third-party apps and other solutions:
Third-Party Apps: Many apps available on the App Store that offer screen mirroring to non-AirPlay compatible devices (like some Samsung, LG, or Roku TVs) require a paid subscription or in-app purchase to unlock full functionality, remove ads, or access premium features like sound and higher resolution.