to me you have to define your own function

Function sum_no_formulas(target As Range) As Double

For Each cell In target
If Not cell.HasFormula Then
sum_no_formulas = sum_no_formulas + cell.Value
End If
Next cell

End Function

this function will sum all cells in a range that include ANY formulae
(i.e. not only those with SUM function but ANY cells strating with =
sign)

check this one out and let me know if it works for you