Amazon ElastiCache for Redis
ElastiCache for Redis User Guide (API Version 2015-02-02)

ElastiCache Reserved Nodes

Reserving one or more nodes may be a way for you to reduce costs. Reserved nodes are charged an up front fee that depends upon the node type and the length of reservation – 1 or 3 years. In addition to the up front charge there is an hourly usage charge which is significantly less than the hourly usage charge you incur with On-Demand nodes. To determine whether reserved nodes would be a cost savings for your use cases, determine the node size and number of nodes you need, estimate the usage of the node, then compare the total cost to you using On-Demand nodes versus reserved nodes. You can mix and match reserved and On-Demand node usage in your clusters. For pricing information, see Amazon ElastiCache Pricing.

For more information, see Managing Costs with Reserved Nodes.