Depending on where your original bucket is hosted, it is very possible that CloudFront ends up costing more in data transfer.

For instance, a us-east-1 bucket has a data transfer rate to the internet starting at $0.09/GB. That's flat, regardless of where the requester is located.

On the other hand, while US- and Europe-based transfer pricing is cheaper in CloudFront (starting at $0.085/GB), all the other regions are more expensive... for example, South America starts at $0.25/GB (!).

Of course the reverse is true: a bucket originally hosted in South America would probably do well to enable CloudFront, as requests from every other region become cheaper...

And yet another wrinkle, I think CloudFront always is cheaper than S3 eventually (with enough data usage). So for very large customers, maybe it is true that CloudFront will always be cheaper. Clear as mud.