I would start by creating a list of discounts. Each item's display text should look something like this:
<input type="hidden" class="discount" value="5"/>
With "5" being replaced with "10," "20," or any other discount price. (You can optionally add internal labels to each item so you don't end up with all this HTML in analysis.)
In your list of attributes, add this HTML to the attribute that represents summed price:
<input type="hidden" id="summed"/>
(Again, I would recommend adding an internal label.)
Now you can create your ACBC exercise using these lists. You'll want to keep the discount attribute from showing up in BYOs, Must Haves, or Unacceptables as respondents won't actually be seeing this attribute.
Finally, add this script to your screener and choice questions:
<script>
$(document).ready(function(){
// Hide discounts
$('.discount').closest('tr').hide();
// Update summed price
$('#summed').closest('tr').find('.level_text').each(function(){
var summedPrice = Number($(this).text().replace(/[^0-9.]/g, ''));
var discount = Number($('td:nth-child(' + ($(this).closest('td').index() + 1) + ') .discount').val());
$(this).text('£' + (summedPrice - discount) + ' per month for 12 months then £' + summedPrice);
});
})
</script>