Option Explicit
Sub Worksheet_change(ByVal Target As Range)
Dim Cout As Range, Neuf_Ancien As Range, Frais As Range, Taux As Range
Application.EnableEvents = False
Set Cout = Range("B1")
Set Neuf_Ancien = Range("B2")
Set Frais = Range("B3")
If Neuf_Ancien = Neuf Then
Set Taux = Range("O1")
Else
Set Taux = Range("O2")
End If
If Cout <> "" And Neuf_Ancien <> "" Then
Frais = 1000 + Taux * Cout
Else
Frais = ""
End If
Application.EnableEvents = True
End Sub