Apple charges $79 for an out-of-warranty battery replacement, fwiw. That includes the labor and risk of damage to the phone, but still seems a little high.

I've had such bad luck with knock-off batteries in general that I really don't want to risk one in my phone.

My iPhone 6 is down to about 83% max capacity (1500/1810 mAh) after 3 years of daily 1-3 full discharges: ~2200 cycles (total guess, I don't know if the phone tracks this, but I can't find it). Seems like a pretty robust part.