IMO it doesn't make sense for SimpleDB to be billed like EC2. An EC2 instance is using resources even when it's idle, so Amazon has to bill for that. Because SimpleDB is (presumably) fine-grained multi-tenant, it should only bill for the time consumed by query execution. It would have been clearer if Amazon priced it at 0.0000000388 dollars per millisecond or something, but then people might scratch their heads over rounding errors.