Are you using jQuery or pure javascript? Because if you are using jQuery you could use jQuery selectors to search the DOM and find the elements.
$('#valores').change(function() {
$('span[class^="moeda"]').hide();
//$(this) would be the combo box and .val() will
//give you the value of the selected item
$('#' + $(this).val()).show();
});
This is all you'd need to do if you used jQuery.
http://jsfiddle.net/dklingman/S8X6k/
@Update
$('#valores').change(function () {
//Get and hold onto all the spans.
//This helps with not needing to search
//the DOM each time, one and done
var spans = $('div[id^="product"] span');
$(spans).hide();
//$(this) would be the combo box and .val() will
//give you the value of the selected item.
//Now all we need to do is search, (filter), the
//set of stored spans for the ones that have an
//id that starts with the selected value from the
//combo box.
$(spans).filter('[id^="' + $(this).val() + '"]').show();
});