Some of the stuff we do: - rewrite urls - auth and permissions check - serve index.html on directory indexes - inject dynamic content in served html files - add CORS headers
> When you’re working with the HTTP response, Lambda@Edge does not expose the body that is returned by the origin server to the origin-response trigger. You can generate a static content body by setting it to the desired value, or remove the body inside the function by setting the value to be empty. If you don’t update the body field in your function, the original body returned by the origin server is returned back to viewer.
https://docs.aws.amazon.com/AmazonCloudFront/latest/Develope...
> If you are using an AWS origin, effective December 1, 2014, data transferred from origin to edge locations (Amazon CloudFront "origin fetches") will be free of charge.
And in general data out from CloudFront ($0.085 to $0.02 in NA & EU) is cheaper than data out from S3 ($0.09 to $0.05 in us-east-2). For the lambda@Edge portion it's $0.60 per 1 million requests while the CloudFront requests are $0.75 to $1.00 per 1 million requests.
Those are public on demand rates and any customers with significant or consistent usage should definitely contact AWS for lower committed/custom prices.
Edit: Disclaimer I'm a principal at AWS, and in the past spent significant time working on CloudFront & Lambda@Edge, but all of the above is public information.