Crosstab - Using CASE statement how do I format the results as currency?

Below is a column definition in a crosstab table.  The CASE statement works only if the result is a string (shown in code below).  I want the results for this column to be formatted as currency.  Ideas and help would be appreciated.


        WHEN Sum([Quantity]) OVER (NextPeriod([Axis.Rows])) is NULL THEN String(Sum([MOR_GE_COST])) 
        WHEN Sum([Quantity]) OVER (PreviousPeriod([Axis.Rows],1)) is null THEN String(Sum([MOR_GE_COST]))
        ELSE "----"
END as [Mix Chg $]