Pricing differences between AWS SimpleDB and DynamoDB

I'm having trouble understanding the pricing differences between SimpleDB and DyanmoDD (and let's say, RDS). From what I understand, with SimpleDB, you pay for the amount of raw data that you're storing every month, and then you pay for machine utilization time whenever your upload or query your data. So, in a given month, if you never query or upload any data, you'll only pay for the data that's sitting there. With DynamoDB and RDS, it seems like you're paying for server capacity-hours all the time, every month (especially RDS?). Is that right?

Thanks.