Cloud Pub/Sub is priced based on the amount of data sent, received, and published in the Pub/Sub. Each operation is billed a minimum of 1 KB, even if the message and data is less than that. You can find the pricing details at https://cloud.google.com/pubsub/pricing.
It is very helpful to track which application is using which topic and which subscription. If needed, use multiple Google Cloud projects to track the billing of Cloud Pub/Sub more effectively. Design the system to be more flexible in granting and revoking access to service accounts that publish and subscribe, and also to the subscriptions.
You can find more information about the quotas for Cloud Pub/Sub in your projects at https://cloud.google.com/pubsub/quotas. The max quota is unlimited, which helps companies to implement real-time analytics and reporting: