JQuery val(）在设置表单值时引发运行时错误

我是Javascript/JQuery的新手，如果我错过了一些简单的东西，我很抱歉。我正在试着做一个按钮，从一个数量计算总数；单价，并用结果填充合计字段。这是在ASP.NET MVC3中使用VS2010完成的。

我的代码将显示"SpoilegeQuantity"answers"SpoileageUnitPrice OK"的值，但无法设置"SpoilageTotalPrice"的值。错误表明我正在尝试为函数结果赋值，但我的理解是，val（）可以用于获取和设置表单字段值。更正或建议？

VS2010视图：

<div class="editor-label">
    @Html.LabelFor(model => model.SpoilageQuantity)
</div>
<div class="editor-field">
    @Html.EditorFor(model => model.SpoilageQuantity)
    @Html.ValidationMessageFor(model => model.SpoilageQuantity)
</div>
<div class="editor-label">
    @Html.LabelFor(model => model.SpoilageUnitPrice)
</div>
<div class="editor-field">
    @Html.EditorFor(model => model.SpoilageUnitPrice)
    @Html.ValidationMessageFor(model => model.SpoilageUnitPrice)
</div>
<div class="editor-label">
    @Html.LabelFor(model => model.SpoilageTotalPrice)
</div>
<div class="editor-field">
    @Html.EditorFor(model => model.SpoilageTotalPrice)
    @Html.ValidationMessageFor(model => model.SpoilageTotalPrice)
    <input type="button" value="Calculate" onclick="calculateTotal()" />
</div>

JS文件：

function calculateTotal() {
// calculate total spoilage price from unit price and quantity
var totalprice = $('#SpoilageQuantity').val() * $('#SpoilageUnitPrice').val();
totalprice = totalprice.toFixed(2);
$('#SpoilageTotalPrice').val(totalprice);
}

渲染HTML：

<div class="editor-label">
        <label for="SpoilageQuantity">Item Quantity</label>
    </div>
    <div class="editor-field">
        <input class="text-box single-line" data-val="true" data-val-number="The field Item Quantity must be a number." data-val-required="Quantity of item lost is required." id="SpoilageQuantity" name="SpoilageQuantity" type="text" value="0.00" />
        <span class="field-validation-valid" data-valmsg-for="SpoilageQuantity" data-valmsg-replace="true"></span>
    </div>
    <div class="editor-label">
        <label for="SpoilageUnitPrice">Unit Price</label>
    </div>
    <div class="editor-field">
        <input class="text-box single-line" data-val="true" data-val-number="The field Unit Price must be a number." data-val-required="Unit price is required." id="SpoilageUnitPrice" name="SpoilageUnitPrice" type="text" value="0.00" />
        <span class="field-validation-valid" data-valmsg-for="SpoilageUnitPrice" data-valmsg-replace="true"></span>
    </div>
    <div class="editor-label">
        <label for="SpoilageTotalPrice">Total Price</label>
    </div>
    <div class="editor-field">
        <input class="text-box single-line" data-val="true" data-val-number="The field Total Price must be a number." data-val-required="The Total Price field is required." id="SpoilageTotalPrice" name="SpoilageTotalPrice" type="text" value="0.00" />
        <span class="field-validation-valid" data-valmsg-for="SpoilageTotalPrice" data-valmsg-replace="true"></span>
        <input type="button" value="Calculate" onclick="calculateTotal()" />
    </div>

错误消息：

Microsoft JScript runtime error: Cannot assign to a function result

编辑：整个js文件：

/*  Copyright 2012 Tom Barrett
This file is part of PFC Logs.
PFC Logs is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
PFC Logs is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with PFC Logs.  If not, see <http://www.gnu.org/licenses/>.
*/
function autoFocus() {
    // Auto-focus on the first text field upon page load.
    $("input[type=text]").first().focus();
}
function calculateTotal() {
    // calculate total from quantity & unit price
    var totalprice = $('#SpoilageQuantity').val() * $('#SpoilageUnitPrice').val();
    totalprice = totalprice.toFixed(2);
    $("#SpoilageTotalPrice").val(totalprice);
}