Accessing rateschedule variable on sales orders & invoices

I've done the &xml=t trick on sales orders & invoices and found a variable called rateschedule on each line item.
The XML looks like this

<rate>43.47826087</rate>
<rateschedule>0</rateschedule>
<rateschedule>45.217391304347824</rateschedule>
<rateschedule>39</rateschedule>
<rateschedule>44.347826086956527</rateschedule>
<rateschedule>79</rateschedule>
<rateschedule>43.478260869565229</rateschedule>

  The above XML essentially describes my pricing structure for that line item where the rate is what the customer was charged and the other values describe pricing for quantity of 0, 39 & 79 respective.

What I'm looking to achieve is to get the value 45.217391304347824 so I can compare to the rate of 43.47826087 and calculate any "discount" to show on my invoices/sales orders.