Is cost of public AMI same as private?

Hi Shantanu Oak,

The charges associated with AMIs do not depend on whether they are public or private. Whether it is public or private does not matter, it depends on the AMI’s storage category for the root device. AMIs are categorized as either backed by Amazon EBS or backed by instance store - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ComponentsAMIs.html#storage-for-the-root-device. With AMIs backed by instance store, you're charged for instance usage and storing your AMI in Amazon S3. With AMIs backed by Amazon EBS, you're charged for instance usage, EBS volume storage and usage, and storing your AMI as an EBS snapshot.
You can also check out the contents of this doc for some more information: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ami-billing-info.html

I hope this helps. Let me know if you have any questions.
Warm regards,
awsamy