I'm afraid thats not possible. The expressions will be shown for each value of COST TYPE. Even if expression returns only nulls for all rows. You could try combining your two expressions into one.
Sum( if(COST_TYPE like 'Frozen',ELEMENT_ITEM_COST, null())) - Sum( if(COST_SUB_ELEMENT like 'MTP',ELEMENT_ITEM_COST, Null()))