that is really greedy of Pantheon, how to escape Pantheon.

i would run it like this, WP2static, generate the blog,

upload to a free ipfs, unlimited bandwidth , these days they provide upto 2gb free hosting, for static files this is lots of power. upload and hook the ipfs hash to your domain.

ipfs could be used with many public gateways, so you could make a forward the user to another cdn.yourdomain.com easily

alternatives is to use, the sia > skynet hosting which is free. and is extremely fast