How much is it to repair a 74% battery life

Get an estimate here (after selecting your country):--> Choose Your Country or Region - Apple Support

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