How much is the cost pf battery of iphone 6s?

The price is approx $ 69.

If you want to know exactly, call Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

or 

call 1-800-MY-APPLE

Using the “Esitmator” here is your best route:

iPhone Battery Repair & Replacement - Apple Support

If available you can begin the replacement process to renew your device right there.