Since calculations may involve multiple attributes, the system does not apply the format defined for any of the attributes to the operation result. If you use a calculation in a report you will need to specify the format for the operation result, for example:
SUM Transaction.Amount WHERE (Transaction IN Account.Transactions), $#,##0.00
However, since the SUM function operates on a single attribute the system could apply the format of that attribute to the function result. This functionality will be added to the system and will be available in the next update. Until then you can specify the desired format as described above.
Remember also that if the format $#,##0.00 is defined for a numerical attribute that is entered on a form, the system will require the user to type the $ sign when entering the value. If this behaviour is undesirable, consider using format #,##0.00 for the attribute and adding the $ sign to the attribute label on forms (for example: 'Amount, $') and other places where the attribute value is displayed such as reports, documents, etc.