Disclosure, I am from imgix.
If you start to exceed $500/month we can offer discounts and certainly when you are looking at millions of images then a flat rate at a lower level makes a lot more sense. 30 million unique images certainly should not be $90,000, that would be outrageous.
The $60 above doesn't include bandwidth from either my transit provider or Imgix, you can purchase a CDN separately.
Imgix is 8c/GB which is certainly competitive but by no means unique. CloudFront is only $0.005 more expensive on the higest tier and gets as low as $0.02 on the lowsest. A cheaper CDN like CDN77 starts at $0.049/GB and goes as low as $0.007/GB.
Cloudfront charges for requests and it can often be the biggest component of your bill.
CDN77 performance in terms of latency is not that great. If you are looking for a cheap CDN with good performance, buy edgecast from a reseller. The only problem with edgecast is that they charge a really high fee for SSL support for custom domains
For me Imgix is primarily a cheap CDN with the benefit of unlimited sizes of each image depending on the device.