As you might know, until the end of 2018, for anyone with an iPhone 6 or later whose battery needs to be replaced (worldwide) Apple offer a reducted price of $29 (work included, taxes excluded).
This is the price in US, in all Apple Stores. If you go to an Apple Authorized Service Provider (AASP), prices may be different.
That being said, you should check the price applied by Apple Stores or AASPs in your country, asking them directly.
Regards
Giulio