HTML:

<div id="box">    
    <p> Valor: <input id="valor"> </p>
    <p> Taxa: <input id="taxa"> </p>
    <p> Imposto: <input id="imposto"> </p>
    <p> Envio: <input id="envio"> </p>
    <p> Taxa adicional: <input id="taxa_adicional"> </p>   
    <p> Subtotal: <span id="subtotal">0</span> </p>
    <p> Total: <span id="total">0</span> </p>   
</div>

JavaScript:

var options = {
        format: '#,###.00',
        locale: 'br'
    },
    inputs = $( 'input:text', '#box' ).get(),
    input_adicional = $( '#taxa_adicional' )[0],
    input_total = $( '#total' )[0],
    input_subtotal = $( '#subtotal' )[0];

and then:

$( inputs ).add( [ input_total, input_subtotal ] ).format( options );

$( inputs ).
    blur( function () {
        var total = 0,
            subtotal = 0;

        // on blur, format the field
        $( this ).format( options );

        // calculate the sum of all input fields
        $( inputs ).each( function () {
            total += +$( this ).parse( options );
        });

        // subtotal doesn't include the "additional" field  
        subtotal = total - $( input_adicional ).parse( options );

        // populate the SPAN's with the sums and format the nubmers
        $( input_subtotal ).text( subtotal ).format( options );
        $( input_total ).text( total ).format( options );
    }).
    focus( function () {
        // if the field contains the value 0, empty it
        if ( +$( this ).parse( options ) === 0 ) {
            this.value = '';
        }
    });

Live demo: http://jsfiddle.net/U9V6x/