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>iFixit is charging $39 for an iPhone mini battery

It is absolutely fine in US. When iPhone Battery Replacement parts isn't even a listed item on Amazon but only Replacement Kits. ( Likely due to whatever agreement they had with Apple ) And Labour cost are high. One may even borderline call Apple's pricing cheap in the US. And why Apple could claim in court they made no profits in their after sales repairing services .

If you live within the Valeriepieris circle, have retail access to slightly higher than BOM of battery. Things might look a little different.

Hopefully we will have Battery improvements in the next 10 years so most of these discussions will become non-issues.