You can change the settings on the Number tab of the properties window of the statistical box. Click on the function in the list on the left and then check the Override Default Settings checkbox and finally choose a format you like.
BUT you can't make the formatting depend on your selections elsewhere in the document. If you want that you'll need to create your own statistical box using a straight table and use the num and/or money functions and expressions that check your selections and apply one format or another.
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