Use this
to find the span. this
inside the event handler refers to the element which called the handler in this case the clicked btn-produto-detalhes
element, so use it along with .find() to find the span
inside it
$('.btn-produto-detalhes').click(function(e) {
e.preventDefault();
$(this).prev('div').slideToggle('fast');
//find the span within current btn-produto-detalhes
$(this).find(span').toggleClass('minus');
});