$29 but they dont have any. But they're happy to have you wait in line for more than an hour to tell you.
You have to order the battery by calling Apple's customer service number. Wait about 30 minutes on hold to get a hold of an actual person who will open your case. Wait about 6 weeks (even though they say 3-5) for battery to get to the Apple store in Walnut Creek. Hope this helps.
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