Text box value based on two other text boxes

Hi could anyone help please?

im trying to calculate a text box based on two different text boxes, so if my combox = eur then it shows the euro rate, if it equals usd it shows the usd rate, this works fine, if i put a figure say 20 in text box 2 once i have clicked through the form, the purchase price divides 20 by the rate, but if i change the combo box to usd it wont change the purchase price?.....

What am i doing wrong here?

Private Sub TextBox2_AfterUpdate()
If (ComboBox1.Value = "EUR") Then
FXRate.Value = ActiveSheet.Cells(1, 6)
End If

If (ComboBox1.Value = "USD") Then
FXRate.Value = ActiveSheet.Cells(2, 6)
End If


End Sub

Private Sub purchaseprice_Enter()

If purchaseprice = "Enter Price Here" Then
purchaseprice.Value = TextBox2.Value / FXRate.Value
Me.purchaseprice.Text = Format(Me.purchaseprice.Text, "£0.00")
End If

End Sub

Thanks Dan