How much is it to fix a back camera on an iPhone 12 Pro Max?

What's wrong with it? If it's not physically damaged, it should be under warranty.

If it is physically damaged, the phone will have to be replaced.

https://support.apple.com/iphone/repair/service

Replacement is US$599 without AppleCare+.

Apple Care or AppleCare+ ? They are not the same thing.

If you have AppleCare+ a replacement phone is $99. If not, it's $599.