I'm wondering about the cost of using AmazonS3 as a CDN. S3 is great for storage (archive/backup) of files, because storage costs are dirt-cheap. However, the bandwidth costs for transferring to/from S3 aren't necessarily so cheap.

For a low-traffic site that doesn't exceed its monthly bandwidth quota, using a CDN makes no sense at all, as it merely adds additional monthly bandwidth costs. For a high-traffic site, the bandwidth costs from the CDN are likely much lower than bandwidth quota increases from the web host.

So, what is the traffic (bandwidth) break-even point, above which a site would start realizing cost savings using S3 as a CDN (or WPCDN, for that matter)? I would think the traffic would have to be in the tens of thousands per day (or, in my case, almost 2 GB of bandwidth use per day).