It's really expensive to do that on top of CloudFlare, and most apps like that aren't using a tremendous amount of bandwidth.
We're not _yet_ a great option as a general purpose CDN replacement (both pricing and featureset) but we're getting there, and are pushing prices down continuously. It's more expensive to buy bandwidth and power in certain cities, too, hence the different pricing per region.
If you're willing, I'd love to know what your traffic breakdown is and how you got to that $1200/mo number. You can email me if you'd like! (kurt.mackey@fly.io)