If you still haven't changed the battery in your older iPhone, the price for Apple has gone back. But if you are interested in doing it yourself, iFixit is still discussing their $ 30 battery insertion rates.
If you are out of the loop, it was discovered just over a year ago that Apple deliberately thrashed iPhones with old, degraded batteries to prevent random shutdowns and other problems. Fix? Replace the battery. But Apple didn't tell anyone until they were caught red hand.
At that time, Apple began offering discounted battery replacement, which also led to increased waiting times at Apple Stores. iFixit penetrated and discounted their battery charge kits so if you didn't want to wait for Apple, you can do it yourself. And while it may seem scary, it's not really that bad with a process.
If you are not sure if your iPhone can use a battery replacement in the first place, check the battery health by going to Settings> Battery> Battery Health. From there, it will tell you if you can take advantage of a replacement.