Change multiple elements with jQuery

Question 1

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();
});

http://jsfiddle.net/dklingman/UBEm2/4/

Question 2

getElementsByClassName is like getElementById.

 var x=document.getElementById("the_id") //getElementById method

 var y=document.getElementsByClassName("the_class") //getElementsByClassName method

You type the class name in just like you'd type the id in.

---Edit:---

document.getElementsByClassName returns an array-like object.

So do something like this:

 var first=document.getElementsByClassName("the_class");

 document.write(first[0]);

*Use document.write for testing purposes only.

Now if you wanted the price to change at the beginning of your script make a variable: var price="R$ 16.90";

Then to change the price do: price*0.95 //5% discount price*0.90 //10% discount price*0.85 //15% discount