How expensive is rsync if nothing has changed?

How "expensive" is an rsync on the various computing resources (CPU, RAM, network, etc.) if nothing has changed on the source file system?

I've two weak servers across a very high latency network and files show up sporadically (due to a process out of my control), but I really need to sync aggressively.

I'm doing a basic recursive send on an entire folder.

How does this change if there are 100s of GBs or maybe TBs of data sitting (again, mostly unchanged) on the source filesystem?

The source may be a CIFS mounted share, which does not work with inotify, which is why I'm asking about rsync performance characteristics, not alternative mirroring solutions.