I appreciate this is an old post but thought it may be worth replying.

IOPS provisioning is optional and if you provision a set value you will be charged for it. You would be better getting a benchmark from your existing server by capturing the reads \ writes per second across your disks and then setting a value accordingly to your server in AWS.

Are you talking an EC2 AMI or RDS?

In my experience we have found that by not provisioning your IOPS you are at the mercy of amazon and how the underlying disk system services the requests.

For example we have an MySQL RDS instance which on an instance with 1000 provisioned IOPS consistently reports disk write latency at 0.003 seconds when performing approx 700 IOPS.

On another RDS instance running the same workload and an exact copy of the other instance it chokes at a peak of 300 IOPS and disk latency increases by over 600%.