As a consumer, why not just order the battery when your battery is down to 80-85% health and it’s about to be convenient to spend a half-hour of quality time with your phone? As repair shop, Apple has already undercut your price to use official supply chain parts, so you’re likely already using the <6RANDOMCAPITALS> branded batteries and the $100 bootleg part programmer.
In my case self-repair is more about convenience than money. Bootleg parts/tools are still a better experience for me than shipping a phone to Apple for repair or making appointments to the Apple Store and then reinstalling the software on the new phone (twice, if you have to do it on a spare device to use in the interim).
Regarding batteries specifically, I agree, Apple's prices are unbeatable. In my case, my main use-case for self-repair is screens - the batteries are just a bonus because if I'm already in the phone I may as well put a new battery in there. The pricing on screens (including the "core charge") is much higher and makes self-repair more attractive.