This page explains pricing for Kubernetes Engine nodes and clusters.
Pricing for nodes
Kubernetes Engine 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 1 minute minimum usage cost.
You can use the Google Cloud Platform pricing calculator to create an estimate of your monthly Kubernetes Engine 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 Kubernetes Engine abstracts away the existence of the master, you are not charged for the master node.