Calculate select field values for a form

You can do this:

$('select').change(function(){
    var sum = 0;
    $('select :selected').each(function() {
        sum += Number($(this).val());
    });
     $("#valorTotal").val(sum);
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script><selectid="precoFruta" multiple name="valores">
        <option value="1.5">Macã - R$1,20</option>
        <option value="2.5">Uva  - R$2,50</option>
        <option value="1.0">Limão- R$1,00</option>            
    </select>
    
    <input type="text" class="form-control" id="valorTotal" name="valorTotal" value="" >

You can run the above code and see the result.

To get all the option selected you have to go one by one and get your value.

Then you can put everything together in a string to show in the input.

You can do it like this:

$('#precoFruta').on('change', function() {
    var values = $(this).find('option:selected').map(function() {
        return this.value;
    }).get().join(', ');
    $('#valorTotal').val(values);
});

jsFiddle: link

Se for caso de quereres a soma deles podes fazer assim:

$('#precoFruta').on('change', function() {
    var soma = $(this).find('option:selected').get().reduce(function(sum, el) {
        return sum + Number(el.value);
    }, 0);
    $('#valorTotal').val(soma);
});

jsFiddle: link