It seems like a bad strategic decision to try to build a CDN on top of a platform that already offers a CDN while expecting to be cheaper than their economy-of-scale-driven CDN.
Not to say that you couldn't build a CDN on AWS, but if you're going to do it, wanting to do it for cheaper than someone who doesn't pay markup on the same instances (Amazon) isn't realistic.
Not to say that you couldn't build a CDN on AWS, but if you're going to do it, wanting to do it for cheaper than someone who doesn't pay markup on the same instances (Amazon) isn't realistic.