MDX - Sum values greater than then a median calculated measure
You need first to define a measure that will compte the median price across the siblings:

[Measures].[Siblings_median_price] 
AS 
MEDIAN([DimItems].[Dim Items Id].CurrentMember.Siblings, [Measures].[Price])


The following measure should return what you want:

[Measures].[Sum_below_median] 
AS 
SUM(
     [DimItems].[Dim Items Id].CurrentMember.Children
   , IIF([Measures].[Price] &lt; [Measures].[Siblings_median_price]
   , [Measures].[Quantity], Null)
)