Google Cloud Pub/Sub is priced according to the data volume used in a calendar month:
|Monthly data volume||Price Per GiB|
Data volume is computed from message and attribute data for pull,
push, and publish operations. The minimum charge per request (not message) is 1 KB of data. For
requests that are smaller than 1 KB, a charge for 1 KB will be applied. For example, if in one month
50 pull operations are requested, each with a size of 500 bytes, the charge for those requests will be
1 KB * 50 requests * price/GiB for the applicable monthly data volume tier.
Note that prices are listed per GiB (230 bytes,
approximately 1.07 billion bytes).
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.