How much is iPhone 6s battery replacement?

iPhone 6s Shutdown Issue

See the following from iPhone 6s Program for Unexpected Shutdown Issues:

Apple has determined that a very small number of iPhone 6s devices may unexpectedly shut down. This is not a safety issue and only affects devices within a limited serial number range that were manufactured between September and October 2015.

If you have experienced this issue, please visit an Apple Retail Store or an Apple Authorized Service Provider and have your device's serial number checked to confirm eligibility for a battery replacement, free of charge.

Note the last line in the article Kappy referenced:

"This program covers affected iPhone 6S batteries for 3 years after the first retail sale of the unit."

So your device may not qualify either because it not one of the affected serial numbers, or because it is more than three years old. Note the iPhone 6S went on sale in September, 2015.

If you need to pay for the battery replacement, and if there is no other damage to the iPhone, and if you are in the U.S., the price would be US$49 plus tax.