Web Forms :: Enable Required FieldValidator When CheckBox Is Checked

I  have textbox and RequiredFieldValidator and checkbox in my page

put textbox and RequiredFieldValidator in div
<div class="CGM2_1">
<div class="CGM2_121">
<asp:TextBox ID="Txtprice" runat="server" CssClass="Txtprice1"></asp:TextBox>
</div>
<asp:Label ID="Label27" runat="server" CssClass="lblprice1"><span style="color:Red">*  </span>قیمت:</asp:Label>
<asp:RequiredFieldValidator ID="RequiredFieldValidator5" runat="server" ErrorMessage="price"
ControlToValidate="Txtprice" Display="None" Enabled="false"></asp:RequiredFieldValidator>

</div>

And make RequiredFieldValidator Enabled="false"

When users checked checkbox it show Div

Below is code:

<script type="text/javascript">
function ShowHideDiv(CHBPrice) {
var CGM2_1 = document.getElementById("CGM2_1");
CGM2_1.style.display = CHBPrice.checked ? "block" : "none";
}
</script>

Now I want when users checked checkbox it does:

 RequiredFieldValidator Enabled="true"

How I can do it in above code?