Validators - RangeValidator Can't Do Even The Most Basic Math?

I'm having an issue with my ASP.NET RangeValidator controls.

I want to allow users to enter a discount amount, and this amount must be negative (< $0.00). I want to verify that the amount entered in a textbox is a negative value, so I have this in my page markup:

<asp:TextBox ID="tbxDiscount" runat="server" />
<asp:RangeValidator ID="rvDiscount" runat="server" ControlToValidate="tbxDiscount"
MinimumValue="0.0" MaximumValue="0.0" EnableClientScript="true"
ErrorMessage="Please enter a negative value for a discount" />

and I attempt to set the MinimumValue dynamically in my code before the page gets rendered - to the negative equivalent of my item price. So if the item is $69, I want to set the minimum value to - $69:

rvDiscount.MinimumValue = (-1.0m * Price).ToString();

Trouble is: I keep getting this error message:

The maximum value 0.0 cannot be less
than the minimum value -69.00 for
rvDiscount

WTF?!?!??! Where I come from, -69 $ IS less than $0 ...... so what's the problem?