A Plan is Dealer price + D&D + $75 admin fee + finance charges.
D plan is Dealer price + D&D + $100 advertising surcharge + $75 admin fee + finance charges.
X plan is a Dealer invoice - 0.4% discount on Fords and Mercurys or + 0.6% premium on Lincolns plus a $75 Admin fee.
Plus whatever rebates are available at the time, I don't really think there's spittin' difference between any of them. A or D plan will get you a better price, I dunno if it'd really be worth fighting for though. I might be wrong; maybe someone better versed in sales (paging 02Supercrew4X...) can break it down better than I.