This page explains pricing for Google Kubernetes Engine nodes and clusters.
Pricing for nodes
GKE uses Google Compute Engine instances for nodes in the cluster. You are billed for each of those instances according to Compute Engine's pricing, until the nodes are deleted. Compute Engine resources are billed on a per-second basis with a one-minute minimum usage cost.
You can use the Google Cloud Platform pricing calculator to create an estimate of your monthly GKE charges, including node pricing.
About Kubernetes master nodes
If you have used the open-source Kubernetes project, you are familiar with the Kubernetes master node running on its own machine instance. Because GKE abstracts away the existence of the master, you are not charged for the master node.