Apple is currently offering, 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.
Apple says the refund will be issued through an electronic funds transfer or as credit on the card that paid for the battery replacement. Customers who are eligible for the refund should hear from Apple through email between May 23 and July 27 on how to obtain the credit.
In December 2017,in order to prevent unexpected shutdowns. Not everyone was happy to hear their iPhone was being artificially slowed. As a remedy, Apple lowered the price of its to $29.
The new price took effect in December, but by then people had already paid full price for a new battery. Back in February Apple said that it would partially refund these people. This latest announcement tells us how Apple plans to follow up.