$( "#amount" ).val( $( "#slider-range-max" ).slider( "value" ) );
и после $( "#amount2" ).val( $( "#slider-range-max2" ).slider( "value" ) );
добавить var result = $( "#slider-range-max" ).slider( "value" ) * $( "#slider-range-max2" ).slider( "value" );
$( "#how-much" ).html(result);
<script type='text/javascript' src='http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js?ver=3.7.1'></script>
<script type='text/javascript' src='http://dftester.16mb.com/wp-content/themes/vulcano/js/script.js?ver=3.7.1'></script>