Google and GKE team member here: The AKS announcement was either poorly worded or intentionally vague. I choose to grant the benefit of the doubt.

For small clusters (less than 1-5 nodes) GKE does not currently charge anything "extra" except your own node VMs. That is LITERALLY a $0 master.

Beyond 5 nodes we currently charge a flat rate that covers your zonal master(s) regardless of how many or how big they need to be.