What makes you think that?
Cloud Run [pricing page](https://cloud.google.com/run/pricing) explicitly says : "charge you only for the resources you use, rounded up to the nearest 100 millisecond"
Also, Cloud Run's [autoscalling](https://cloud.google.com/run/docs/about-instance-autoscaling) is in effect, scaling down idle instances after a maximum of 15 minutes.
(Cloud Run PM)