I'm not too sure about this that you would find it "great". That's because conditional formats aren't what they appear. You wouldn't be able to do without vba - and even then it's quite difficult - any calculations with the value becuse its only a format. It's just some kind of fooling user's eyes, or in other terms conditional formats are another level you aren't supposed to access.
Just in case you will give it a shot with vba, open a new file and write a number <> 0 into cell A1. Copy the code to the modul of the sheet:
Code:
Private strselected As String
Private dblOldValue As Double
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Const strRange As String = "A1"
If Not Intersect(Target, Range(strRange)) Is Nothing And Target.Count = 1 Then
dblOldValue = Target.Value
strselected = Target.Address
End If
End Sub
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = strselected Then
strselected = ""
Target.Value = Target.Value * dblOldValue
End If
End Sub
Now every time you enter a number in cell A1 the old value will be multiplied by the entered value.