How much will it cost me to replace my battery

$69 (U.S.) at the Apple Store or an authorized Apple service shop. I do not have cost estimates for other countries.

You can find a list of authorized shops in your area by using this link.......Find Locations

The cost of iPhone battery replacement can vary depending on the country or region you reside in. A few factors contribute to this price for replacing your iPhone's battery.

Apple Inc can replace your iPhone battery for a service fee. Their warranty doesn't cover batteries that wear down from normal use. 

iPhone Battery Repair & Replacement