How much would it cost me to replace the battery of iPhone 7plus
iPhone 7 Plus battery replacement
iPhone 7 Plus, iOS 15
Check here: iPhone Battery Repair & Replacement - Apple Support
If you are going to replace the battery, by Apple or an Apple Authorized Service Providers read this Apple article:
Get your iPhone, iPad, or iPod touch ready for service
Get your iPhone, iPad, or iPod touch ready for service - Apple Support