S3's standard tier costs a fifth of ECR in terms of costs per GB stored. Egress costs to the free internet are the same, with the exception that for public ECR repositories they make egress to inner-AWS usage free.

The cost is the S3 cost though. It depends on region and storage tier, but the storage cost per GB, the GET/PUT cost, and the bandwidth cost can be found on the AWS website: https://aws.amazon.com/s3/pricing/