It's not. But the cost difference between servers and serverless is maybe exponential. As traffic gets really high, serverless is going to be extremely expensive compared to a dedicated box.
Definitely something I look out for, but the advantage over something like Cloud Functions, which would be an alternative, is that a Cloud Run service handles multiple requests per instance so it isn't 1:1 with request rate.
No, the concurrency of GCF is always 1. If the latency of your code handling request is 1s, to handle 100 requests per second you'll need to have 100 instances running.
I don't understand what would make it exponential as opposed to merely linear with traffic.
There's nothing in their pricing documentation to suggest this.