If you haven't managed to sort this yet, here's one solution.
I'd use a combobox for pre1unit with 2 columns, set a value list 1000000, "Mega", 1000000000, "Giga" etc. Hide the first column by setting its size to 0cm in Columns Widths. Set Limit to List to Yes.
Set the AfterUpdate events of the textbox and combobox to calculate the prevalue:
Private Sub UpdatePreValue()
Me.TextPreValue.Value = Me.TextPre1.Value * Nz(Me.ComboPre1Unit.Value, 0)
I hope that helps you out.