Each request made to SimpleDB returns a BoxUsage value as part of the response. This response indicates the amount of system resources that were utilized by that specific operation. This value can be used to figure out the cost of making any kind of a request to SimpleDB, and can thus be used to tune queries.
The value of BoxUsage is always consistent for operations such as creating a domain, but in the case of Select requests that are usually used for querying SimpleDB, the BoxUsage values are normalized and are supposed to reflect two parameters that categorize your SimpleDB data:
Your dataset and the data that is contained within it
The complexity of the query that you are making to SimpleDB for retrieving your data
This enables one to optimize and tune their queries using the BoxUsage value. This value is not affected by what other users are doing when utilized in the context of making Select queries...