You can use the BottomCount function to return the bottom n members from a set for a supplied measure.
The query below uses the BottomCount function to return the bottom 10 non empty products with the lowest Sales Amount.
SELECT {[Measures].[Sales Amount]} ON 0,
BOTTOMCOUNT(NONEMPTY([Product].[Product].[Product].Members, [Measures].[Sales Amount]), 10, [Measures].[Sales Amount]) ON 1
FROM [Adventure Works]