You cannot bind a control to the sum of another control. As far as I know, you can't sum another control. If you know the data source for your combo box, could you not define a similar SUM query to calculate the total? e.g. bind your field to
=DSum("MyField","MyTable","MyCriteria"
You can reference any column of a combobox if you want using the Column(Index) property. You could also iterate through all rows in the combobox if you want e.g.
Dim Total As Long
Dim A As Long
For A = 0 To cboMyCombo.ListCount - 1
Total = Total + cboMyCombo.Column(1,A)
Next A
... so what you could maybe do is to make the above code a public function, and then bind your textbox control to it.
e.g.
Public Function SumComboColumn(TheCombo As ComboBox, TheColumn As Integer) As Long
Dim Total As Long
Dim A As Long
For A = 0 To TheCombo.ListCount - 1
Total = Total + TheCombo.Column(TheColumn, A)
Next A
SumComboColumn = Total
End Function
Then make your textbox ControlSource
=SumComboColumn(MyComboBox,1)
Will that do?