There are two components in Amazon EMR billing, the cost for the EC2 instances launched by the cluster and the charge Amazon EMR adds for managing the cluster. When you use Spot Instances, the Spot Price may change due to fluctuations in supply and demand, but the Amazon EMR rate remains fixed.
When you purchase Spot Instances, you can set the bid price only when you launch the instance group. It can’t be changed later. This is something to consider when setting the bid price for an instance group in a long-running cluster.
You can launch different instance groups at different bid prices. For example, in a cluster running entirely on Spot Instances, you might choose to set the bid price for the master instance group at a higher price than the task instance group since if the master terminates, the cluster ends, but terminated task instances can be replaced.
If you manually start and stop instances in the cluster, partial hours are billed as full hours. If instances are terminated by AWS because the Spot Price rose above your bid price, you are not charged either the Amazon EC2 or Amazon EMR charges for the partial hour.
You can look up the current Spot Price and the on-demand price for instances on the Amazon EC2 Pricing page.