For cost optimization, you have to select the appropriate purchasing instance option, and the right instance type for your application workload. As we have discussed in detail about different instance types, you can select the right one to optimize performance.
Once you finalize the instance type, then you have the option to purchase a reserved instance. This is an upfront commitment but reduces costs drastically.
Reserved instances are offered for 1 year or 3 year commitments. If your requirements change before this time period, then you can use the EC2 Container Service to increase instance usage or sell your reserved instance into the AWS marketplace.
You can also use Auto Scaling to reduce the cost. Let's say your workload runs during business hours, then you can configure Auto Scale to launch new and appropriate instances for that known predictable load, and reduce the instance by Auto Scaling after office hours.
You can use spot instances. Spot instances...