Are we already charged at Cloud CDN rates for static and cache-control: public content? Or do the flat bandwidth rates apply for CDN egress? Cloud CDN can be attached to the GCE load balancers, so it stands to reason that GAE already runs on it.
This pricing documentation states that:
When Cloud CDN serves your content, you’re charged for bandwidth and HTTP(s) requests. Cache fill and cache egress charges apply to cacheable content. On cache hits, you pay for cache egress bandwidth. On cache misses, you additionally pay for cache fill bandwidth. These charges replace the network egress charges that apply when serving directly from Compute Engine or Cloud Storage.