Computed fields not updating in UI

Public ReadOnly Property TotalMonthlyPayment()

	Return MonthlyPayment.GetValueOrDefault + OptionPayment.GetValueOrDefault + UpgradeAmount.GetValueOrDefault
End Sub

If I changed one of the 3 values in my UI the total field is not updated.  I found this article and it seems like it should be.
http://www.ideablade.com/DevForceClassic/TechTips/techtip_refreshing_dependent_parents.htm
Specifically here:
"The answer depends upon whether the independent property that got changed lives in the same object, or an external one. Suppose, for example, that you have an Employee class that defines a (computed) Age property whose values depends upon the Employee’s (persistable) BirthDate (as illustrated in many of our Fundamentals tutorials). If a user changes the BirthDate value, PropertyChanged will fire on the Employee instance. Data bindings to that Employee in a form or UserControl listen for this event, and they will notify their controls when it occurs. All the data bindings for the Employee instance will get updated, so the Age value will be updated to correspond to the new BirthDate."

If I'm reading this right I shouldn't need to do anything for my computed value to update but that's not what I'm seeing.



I did write this and it works but it seems like a lot of work for simple calculated fields, am I missing something?

Partial Public Class Order
  Inherits ibem.entity
 

  Private _TotalMonthlyPayment As Decimal
  Private TotalInit As Boolean = False
 

  <AfterSet(EntityPropertyNames.MonthlyPayment)> _
  <AfterSet(EntityPropertyNames.OptionPayment)> _
  <AfterSet(EntityPropertyNames.UpgradeAmount)> _
  Public Sub FireTotalMonthlyPaymentChanged(ByVal args As PropertyInterceptorArgs(Of Order, [String])) 
    CalulateTotalMonthlyPayment()
    OnPropertyChanged(New PropertyChangedEventArgs("TotalMonthlyPayment"))
  End Sub
 
  Public Sub CalulateTotalMonthlyPayment()
    _TotalMonthlyPayment = MonthlyPayment.GetValueOrDefault + OptionPayment.GetValueOrDefault + UpgradeAmount.GetValueOrDefault
  End Sub
 
  Public ReadOnly Property TotalMonthlyPayment() As Decimal
    Get
      If TotalInit Then
        Return _TotalMonthlyPayment
      Else
        CalulateTotalMonthlyPayment()
        Return _TotalMonthlyPayment
      End If
    End Get
  End Property