<tr>
    <input type="text" name="quantity_box[0] class="form-control">

    <input type="text" name="unit_price[]" class="form-control">

    display in
    <input type="text" name="price[]" id="price" class="form-control" autofocus="">
</tr>
<tr>
    <input type="text" name="quantity_box[] class="form-control">

    <input type="text" name="unit_price[]" class="form-control">

    display in
    <input type="text" name="price[]" id="price" class="form-control" autofocus="">
</tr>