Pricing changes noted in this document are in effect as of January 20, 2017, and will be reflected starting with your February 2017 bill. You can read the old pricing guide here to get a better sense of what has changed. If you have any questions or concerns, contact Google Cloud Billing Support or your Account Manager.
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.