I dunno I put a lot of traffic through Vercel, maybe 100k visitors per day, and it was under a few hundred a month. I think a couple EC2 instances behind a load balancer would cost similar or more. I was under the impression that its still a VC subsidized service.
They regularly try to get me to join an enterprise plan but no service cutoff threats yet.
You could probably serve 100k visitors from a $5 VPS, depending on the application.
That said, I understand people are paying for basically not having to think about infrastructure, and agree that that's theoretically worth money, if they could do it well.