Rubygems.org AWS bill for Feb 2014 [pdf] 12 years ago
With most of this being bandwidth costs, it seems like switching to a host like Digital Ocean would make more sense here. The bandwidth costs are a fraction of Amazon's in comparison.
As for the CDN, switching to something like Cloudflare might make more sense rather than relying on Cloudfront. At the least, there's a "US and EU only" option for edge locations to use which si considerably cheaper than the default option of all edge locations.