Sono riuscito a fare questo:

codice HTML:
<select name="prova[]">
<option data-id="1" value="50">Single</option>
<option data-id="2" value="225">Single2</option>
</select>

<select name="prova[]">
<option data-id="1" value="15">Single3</option>
<option data-id="2" value="35">Single4</option>
</select>

<span></span>

function sum(x, y) {
     var z = x + y;
    return z;
}
function calculate() {
    var prezzo = [];
    var num = [];    
$('select').each(function ()        {
             prezzo.push($(this).val());
             num.push($(this).data('id'));
        });
    $('span').text('Totale: '+ sum(prezzo, num));
}

calculate();

$('select').change(function() {
 calculate(); 
});
Ma non mi calcola il totale