Google Cloud Pub/Sub is priced according to the data volume used:
|Monthly data volume||Price Per GB|
Data volume is computed from message and attribute data for pull, push, and publish operations. A minimum of 1KB of data per request (not per message) will be assessed.
If you are using Pub/Sub across projects, note that Pub/Sub fees are billed to the project that contains the requested resource (e.g., subscription). For example, a service account in project A may be given subscriber access to a subscription in Project B (that is under billing account B). Billing account B will be billed for the data that is pulled from the subscription by service account A. Alternatively, if the subscription resides in project A (even if it is attached to a topic in project B), account A will be billed for data that is pulled from the subscription.
For a complete, authoritative list of Pub/Sub prices see Cloud Platform SKUS.