I have this form where the user can insert a quantity in an input field, and see the total in another input field. It works when you insert the numbers manually and I want it to work with buttons too, but i just can't get it to work.

Here's the code:

Code HTML4Strict:
<form id="buyForm" method="post" action="cart.php">
    <label>Choose quantity</label>
        <a href="#" class="button">(+)increase</a>        
        <input type="text" id="qty1" name="qty[]"/>
        <a href="#" class="button">(-)decrease</a>
    <input type="text" id="cost1"  value="50" style="display:none; visibility:hidden;" name="cost[]" />
    <input type="text" id="price1" name="price[]" />

Code JavaScript:
// Calculate
function calc(idx) {
  var price = parseFloat(document.getElementById("cost"+idx).value)*
  //  alert(idx+":"+price);  
  document.getElementById("price"+idx).value= isNaN(price)?"0.00":price.toFixed(2);
window.onload=function() {
  document.getElementsByName("qty[]")[0].onkeyup=function() {calc(1)};
  document.getElementsByName("cost[]")[0].onkeyup=function() {calc(1)};
//Increase/decrease buttons
$(function() {
    $(".button").click(function() {
        var $button = $(this);
        var oldValue = $button.parent().find("input").val();
        if ($button.text() == "+") {
          var newVal = parseFloat(oldValue) + 1;
          // AJAX save would go here
        } else {
          // Don't allow decrementing below zero
          if (oldValue >= 1) {
              var newVal = parseFloat(oldValue) - 1;
              // AJAX save would go here

Here it is as jsfiddle: http://jsfiddle.net/rcheH/5/

Can someone please help me with this?

Thanks in advance!