I want to extend sort by drop down for special price or discounted product from toolbar.phtml
<div class="sort-by" style="float:left">
<fieldset class="sort-by">
<label><?php echo $this->__('Sort by') ?></label>
<select onchange="setLocation(this.value)">
<option value="<?php echo $this->getOrderUrl('price', 'asc') ?>"<?php if($this->isOrderCurrent('price') && $this->getCurrentDirection() == 'asc'): ?> selected="selected"<?php endif; ?>> Price - Low to High </option>
<option value="<?php echo $this->getOrderUrl('price', 'desc') ?>"<?php if($this->isOrderCurrent('price') && $this->getCurrentDirection() == 'desc'): ?> selected="selected"<?php endif; ?>> Price - High to Low </option>
<option value="<?php echo $this->getOrderUrl('name', 'asc') ?>"<?php if($this->isOrderCurrent('name') && $this->getCurrentDirection() == 'asc'): ?> selected="selected"<?php endif; ?>> Name - A to Z </option>
<option value="<?php echo $this->getOrderUrl('name', 'desc') ?>"<?php if($this->isOrderCurrent('name') && $this->getCurrentDirection() == 'desc'): ?> selected="selected"<?php endif; ?>> Name - Z to A </option>
<option value="<?php echo $this->getOrderUrl('update_at', 'desc') ?>"<?php if($this->isOrderCurrent('update_at') && $this->getCurrentDirection() == 'desc'): ?> selected="selected"<?php endif; ?>>Newest First</option>
</select>
</fieldset>
</div>
How I can add one more option for discount so products will sort by special price ASC?