[Dilettante_Pro]

Более надежный способ

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
<script>
    $(function () {
        $('.countprod').on('change', function () {
            var itemId = $(this).attr('id');
            itemId = itemId.substr(itemId.indexOf('_') + 1);
            var newCnt = $(this).val();
            var itemPrice = $('#itemPrice_' + itemId).attr('value');
            var itemRealPrice = newCnt * itemPrice;
            $('#itemRealPrice_' + itemId).html(itemRealPrice);
        });
    });
</script>

<input id="itemCnt_11" type="text" class="countprod" value="1" />
<span id="itemPrice_11" value="123.25">123.25</span>
<span id="itemRealPrice_11" >123.25</span>