iFixit's Do-it-Yourself iPhone Battery Replacement Kits to Remain $29.99 Until End of 2019
The repair website lowered the price of its kits in late 2017 to match Apple's discounted iPhone battery replacement fee of $29, which ended on December 31, 2018. Apple now charges $49 to replace the battery in the iPhone 6 through iPhone 8 Plus and $69 for the iPhone X and newer outside of warranty.
Apple had lowered its battery replacement fee after controversy erupted about a performance management feature it quietly introduced in iOS 10.2.1. The feature, when enabled, has the potential of slowing down older iPhone models with degraded batteries when necessary to prevent unexpected shutdowns.
iFixit's kits include all of the tools necessary to open up an iPhone and swap in a new battery for those willing to give it a try. If the idea of opening up your iPhone sounds uncomfortable to you, it is probably best to stick to the Genius Bar.
Note that do-it-yourself iPhone battery replacements can have warranty implications.