Try something like:

with member [Measures].[Monthly Adjusted Cost] as

IIF(Exists([Calendar].[YearMonthLabel].CurrentMember, {null:[Calendar].[YearMonthLabel].&[201101]}).COUNT > 0

, [Measures].[Cost AMT]*0.95

, [Measures].[Cost AMT])

select non empty [Calendar].[YearMonthLabel].[YearMonthLabel].Members on rows

, {[Measures].[Cost AMT], [Measures].[Monthly Adjusted Cost]} on columns

from [CostCube]

It may be simpler to add a scope statement in your cube rather than doing a calc

scope {null:[Calendar].[YearMonthLabel].&[201101]};

[Measures].[Cost AMT] = [Measures].[Cost AMT]*0.95;

end scope;