Cost of at rest encryption in S3

I am a volunteer for a church. We are investigating using Amazon S3 to store backup data. We have a pretty good handle on how much the storage for this backup data would be, but we also want to encrypt the data while at rest in S3. I have read a bunch about encryption charges, but am still a bit fuzzy on how to compute these costs. From what I have gathered so far, there is no charge for encrypting data while at rest in S3 if we use Amazon managed keys. Is this correct? If not, would someone summarize the factors to consider when estimating the costs of at rest encryption in S3? Thanks.