Maintaining my original value through postbacks and Radio Button events

I have a Radio Button SelectedIndexChanged event that adds 30% to a price if the relevant radio button ("Yes") is clicked.

However, if the person clicks the other radio button ("No") I want the price to revert back to the orginal price.

I cant seem to find a way to store the original price in the vewstate. The method below doesnt work properly as the viewstate takes the price from the actual textbox control (and this gets updated and increased everytime the user selects "Yes" in the Radio Button control. I need to store the price in the view state just once when the method is first run. I guess I need to use some kind of "counter" device that stores the number of times the Radio button has been selected.

    Public Sub RadioButtonList1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles RadioButtonList1.SelectedIndexChanged

        ViewState("OriginalPrice") = lblPrice.Text

        If RadioButtonList1.SelectedItem.Text = "Yes" Then
            lblPrice.Text = FormatCurrency(CInt(lblPrice.Text) + ((CInt(lblPrice.Text) / 100) * 30), 2)
        Else
            lblPrice.Text = FormatCurrency(ViewState("OriginalPrice"))
        End If

    End Sub