https://www.cnet.com/news/apple-partial-refund-iphone-battery-replacement/

Apple is currently offering $29 iPhone battery replacements, but if you paid full price ($79) for an out-of-warranty replacement before Apple discounted its rate, you may get your $50 back.

Apple announced that it's giving $50 refunds to customers who paid full price for an iPhone battery replacement, for iPhone 6 or later, at an authorized Apple store between Jan. 1, 2017 and Dec. 28, 2017. This brings the price down to $29, which is the same it would cost to get a replacement now.