RequiredFieldValidator Based On dropdownlist's value

Hi,
consider we have a dropdownlist like below
<asp:DropDownList ID="drp_WithOwner" runat="server" AutoPostBack="True"> <asp:ListItem Value="-1">--Please Choose--</asp:ListItem> <asp:ListItem Value="1">Sale</asp:ListItem> <asp:ListItem Value="2">PreSale</asp:ListItem> </asp:DropDownList> and we have a TextBox like below
<asp:TextBox ID="txt_Price" runat="server"></asp:TextBox> Is there any way we validate textbox with RequiredFieldValidator only when dropdownlist's value is 2?
Thanks!