adding bootstrap class input-group-addon with TextBoxFor Html Helper

I am trying to add $ sign using bootstrap class 'input-group-addon' with mvc Razro TextboxFor but its not working. Below is my code.

<div class="form-group">
                    @Html.LabelFor(x => x.PaymentAmount, new { @class ="control-label col-sm-2" })
                    <span class="input-group-addon">$</span>
                        @Html.TextBoxFor(m => m.PaymentAmount, new { @class = "form-control col-sm-10" })
                    @Html.ValidationMessageFor(m => m.PaymentAmount, "", new { @class = "help-block" })
                </div>