Battery replacement

Depends on the country. That would be $49 plus tax in the United States through Apple. An Apple Authorized Service Provider can set its own price but they tend to charge about what Apple does. It's different in other countries.

iPhone Battery Replacement - Official Apple Support

There are other limitations that might depend on where it was purchased and where you seek service (gray market devices can be restricted).

▷ iPhone Battery & Power Repair.

Out-of-warranty older iPhone battery = $49 U.S. Dollars.