I’m in the same boat. I also moved to CloudFront not too long ago and the costs are looking similar. I had thought there was some sort of savings due to the local caching, i.e. the bandwidth would be less if the same image repeatedly loaded from the same geographical location, but so far it looks like not.
It seems like there should be some other CDN that should work in place of CloudFront, all Discourse is doing is rewriting image URLS so it is pretty generic (it seems). Maybe Cloudflare could be used for the images only.