Use a RegularExpressionValidatror as follows; with ValidationExpression="\d*\.\d{0,2}"

<asp:TextBox ID="TextBox1" runat="server" Height="22px" ></asp:TextBox>
    <br />
    <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" 
        ErrorMessage="Only 2 digits after decimal" 
        ValidationExpression="\d*\.\d{0,2}" ControlToValidate="TextBox1" 
        Font-Size="10pt" ForeColor="Red"></asp:RegularExpressionValidator>