According to the search results, the cost to fix a battery issue on an iPhone 15 Pro Max without AppleCare+ is $99:
- Apple's battery replacement fee is set at $99 for all four iPhone 15 models, which is unchanged from the iPhone 14 series[1].
- AppleCare+ includes battery service or a replacement battery if your iPhone retains less than 80% of its original capacity[2].
So in summary, if your iPhone 15 Pro Max battery is not holding a charge and you don't have AppleCare+, Apple will charge a $99 out-of-warranty fee to replace the battery. This is the same price as for older iPhone models.
With AppleCare+, battery service is included at no additional cost if your battery retains less than 80% of its original capacity. But for a standalone battery replacement without any other damage, the $99 fee applies whether you have AppleCare+ or not.
Citations:[1] https://www.macrumors.com/2023/09/17/iphone-15-pro-cracked-glass-repair-fees-lower/
[2] https://www.consumerreports.org/money/extended-warranties/is-applecare-plus-worth-buying-for-your-iphone-a8095516790/
[3] https://www.cnet.com/tech/mobile/iphone-15-pro-and-pro-max-will-cost-a-lot-less-to-repair-cracked-back-glass/
[4] https://www.apple.com/support/products/iphone/
[5] https://www.reddit.com/r/iPhone15Pro/comments/17siza8/apple_care_for_iphone_15_pro/