What is the cost of iPhone 7 battery in india

Call Apple in your region see: Contact Apple for support and service

https://support.apple.com/en-us/HT201232

For an indication see this Apple article: iPhone Battery & Power Repair

https://support.apple.com/iphone/repair/service/battery-power

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              

https://support.apple.com/en-us/HT201557