You can use the FORMAT_STRING property to apply formats to calculated members.
The query below shows two calculated members with identical values. However, the FORMAT_STRING property has been applied to the [Measures].[Formatted] calculated member. You can see by setting this property to currency, the returned value is in the currency format.
WITH
MEMBER [Measures].[Unformatted] AS
AVG([Measures].[Sales Amount])
MEMBER [Measures].[Formatted] AS
AVG([Measures].[Sales Amount]), FORMAT_STRING="Currency"
SELECT {[Measures].[Unformatted], [Measures].[Formatted]} ON 0
FROM [Adventure Works]