The subscriptions do not count against your event limit, but do count as cellular data, if the device is online, as ScruffR said. From the pricing page:
An event is any single outbound message from your fleet that exits the Particle Cloud. For example, this could include a Particle.publish() call on a device that triggers a webhook, an integration such as IFTTT, or a server-sent event (SSE).
The following events will not count toward your event threshold:
- Particle.function() calls
- Particle.variable() checks
- Any Publish events that remain wholly within the Particle cloud
The subscription falls into “wholly within the Particle cloud” category.
If you’re generating the events externally and sending to a device via the Cloud API, that’s considered an inbound event, and also not counted against the event limit.