I'd venture to say the batteries are even less than $10/each.My guess is these batteries cost Apple maybe $10 a pop? But like you said, Apple warranties their work, usually done by trained technicians, I can see why they'd charge $80.
As for Apple's previous $79 charge to replace a battery, if people pay that, then that's a fair price. Even if the battery was $4, the tech's time is $10 ($20/hour, 30-minute job), factor in training costs, store overhead, etc, the total cost to Apple for a battery replacement is still way, way, way below $79. Hell, Apple may still make a small profit at this $29 rate they're going to charge. Or maybe it's a break-even cost or a small loss (a really, really small loss).
People commenting about "price gouging" and similar would be best to not operate a business of selling goods. You don't buy an item for $5 and sell it for $6 or $7. Well, you can, but you a) better sell millions of those items or b) you won't be in business long.