iPhone 7 battery replacement cost

Not sure of your location, but from Apple it's $49 plus tax in the United States, and the warranty on all Apple repairs and repair parts is 90 days or the remainder of any warranty, with the exception of Mac batteries with a warranty of 1 year. These are the Apple retail store terms for the United States, but they're similar to those of an AASP or Apple Stores (or mail-in Apple service) around the world.

Legal - Retail Store Service - Apple

6. Apple warrants for a period of ninety (90) days from the date of service (1) that service will be performed in a competent and workmanlike manner and (2) that all parts used to service your product will be free from defects in materials and workmanship, unless otherwise specified by Apple. Apple further warrants to the extent permitted by law, that batteries installed as part of Apple’s battery replacement service for Apple portable Mac computers will be free from defects in materials and workmanship for one (1) year from the date of service. 

saikrishna48 wrote:

Location: India

Don't have a solid idea other than estimates. If it's an Apple Authorized Service Provider, the price varies as each business is allowed to set its own price and procedures. Some might even charge a diagnostic fee (which is actually quite simple and shouldn't really cost anything) before proceeding.

Iphone 7 battery price