Price for changing my battery of iphone 7 plus

See this price list, valid in US Apple Stores (tax excluded, work included):--> iPhone Battery Replacement - Official Apple Support

If you're outside the USA, please check those prices in your country.

Make an appointment in advance and have your battery replaced at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector

Do not forget to create a full backup of your iPhone, before having it serviced.

Regards

Giulio

iPhone Battery Replacement - Official Apple Support