Steve, there is no way to "calculate" a holdback. It varies by manufacturer, by model and by time. Some models have no holdback.
As I stated, you can get all the pricing info for free, but none of the sites I mentioned gives you any info on holdbacks (unless something changed in the last year or so).
IMHO when you are spending some $80,000, saving $15 and not going in fully armed is the height of being penny wise and pound foolish.
Well this will probably get me kicked out of here already!