[devote]

<label><input type="checkbox" data-price="5" /> Книжки</label><br />
<label><input type="checkbox" data-price="3" /> Сумка</label><br />
<label><input type="checkbox" data-price="10" checked /> Ручка</label><br />
<label><input type="checkbox" data-price="13" /> Карандаш</label><br />
<label><input type="checkbox" data-price="27" /> Что-то</label><br />
<div id="total">0 руб.</div>
<script>

    var total = 0,
        totalDiv = document.getElementById( 'total' ),
        els = document.getElementsByTagName( 'input' );

    for( var i = 0, el; el = els[ i++ ]; ) {
        if ( el.type == "checkbox" && el.getAttribute( 'data-price' ) ) {
            total += el.checked ? ( parseFloat( el.getAttribute( 'data-price' ) ) || 0 ) : 0;
            el.onclick = function( e ) {
                var target = e && e.target || window.srcElement;
                total = target.checked ?
                    total + ( parseFloat( target.getAttribute( 'data-price' ) ) || 0 ) :
                    total - ( parseFloat( target.getAttribute( 'data-price' ) ) || 0 );
                totalDiv.innerHTML = total + " руб.";
            }
        }
    }

    totalDiv.innerHTML = total + " руб.";
</script>