[рони]

var x = checkbox[i].val();

вариант...

<input type="checkbox" value="1000" checked="" />
<input type="checkbox" value="2000" />
<input type="checkbox" value="3000" checked />
<script src="http://code.jquery.com/jquery-latest.min.js"></script>
<script>
var checkboxes = $('input[type=checkbox]:checked');
for (var i=0; i < checkboxes.length; i++){
   var x = checkboxes.eq(i).val();
   alert(x);
}
</script>