<select class="input-select" >
<?foreach($arResult['SIZES'] as $id => $size):?>
<option value="<?=$id?>"><?=$size?></option>
<?endforeach;?>
</select>
<select onchange="showPrice(this);" class="input-select" >
<?foreach($arResult['SIZES'] as $id => $size):?>
<option value="<?=$id?>" data-price><?=$size?></option>
<?endforeach;?>
</select>
function showPrice(e) {
$('.total_cost').html(e.options[e.selectedIndex].getAttribute('data-price'));
}