$79 is pretty bad.
$79 every few years on a $500 set of headphones sounds plausible, the battery alone would probably be $40-$50 retail anyways; I'm just getting someone at an Apple store to replace it for me.
More concerning is that a set of new ear muffs costs $69. I've already replaced the ones on my QuietComforts two times ($20 using clones on Amazon), so that is going to add up a bit.
You can buy a pack of 12 rechargeable AAA for less than $10. A charger is $5.
Plus, it doesn't really matter the type of battery. You can find pretty much any of rechargeable battery (3.7V, 7.4V, nickel, Li-ion) for less than $15, retail. It's mind boggling to think that people on Hacker News (of all places) think that is sensible to subject yourself to something as anti-consumer and as environment-unfriendly as what Apple offers.