Ah, Amazon Web Services. One of the biggest moneysinks available. Very good, but not always cheap. Actually, almost never cheap, and always difficult to estimate expenses.
An IO request is a disk-operation. Be that a read, or a write, you're basically being charged with how much data you're getting off their system. It's insane.
You can estimate the IO Requests you're making, by looking at iostat.
800,000,000 IOrequests in 7 days.. That's ~=1300 IOPS . Wow. That's some wickedfast performance.
As an example, a medium sized website database might be 100 GB in size
and expect to average 100 I/Os per second over the course of a month.
This would translate to $10 per month in storage costs (100 GB x
$0.10/month), and approximately $26 per month in request costs (~2.6
million seconds/month x 100 I/O per second * $0.10 per million I/O).
From http://aws.amazon.com/ebs/ about 90% of the way down the page
If you really want to reduce what this is costing you, it might be time to look at whether RDS is really the best fit for your application and project budget.
If you've only got 6k hits a day, you might do better on cost and performance with a dedicated server (preferably 2 of em, one for DB, one for web services).
It's actually a bit more difficult to make that call as you've not said too much more about your application.