> Interesting to see that the pricing is approximately double that of S3, for what I imagine is not much more than a thin layer on top of it.

There's a lot of necessary complexity in the backing platform. Encrypted package blobs are stored in S3 but there are a bunch of other distributed systems for doing things like package metadata tracking and indexing, upstream repository management, encryption, auditing, access control, package manager front-ends, etc... that are not immediately obvious and add cost. The platform that backs CodeArtifact is far from what I'd call a thin layer on top of S3. There is also a team of humans that operate and expand the platform.

Source: I lead the technical design for the product as well as a chunk of the implementation but left the team around mid-2018.