If you were doing the volume of traffic necessary to get the Akamai discount in the range you are suggesting then you are also going to be able to negotiate Amazon pricing in a similar way. It is unfair to compare theoretical bulk CDN pricing to non-discounted Amazon pricing. It would be like negatively comparing the published price of some other CDN to your private knowledge of deals you did with Akamai that probably included contractual commitments.

If you go by published CDN pricing (I use Fastly as an example in another post below) and assume you are pushing close to the maximum bitrate then the comparison is actually reasonably fair. Anything outside of that is speculation based on theoretical deals you might negotiate based on theoretical usages.

However, I would have no problem agreeing that building your own system to handle large scale would probably get to a lower $/GB. But then you have to build it, maintain it, monitor it, etc.