From the website where you register to get your X-plan PIN#, you can go directly to an inventory search, by dealer, and it will show you the X-plan price. I realize you might be trying to figure out how exactly they arrive at the X-plan price, but frankly that's not very useful information. You can find the invoice price (with specific packages, options, etc.) at other websites, and of course the dealer could show you the invoice for a given vehicle on their lot, but what's truly most helpful is that website where you register. Not only can you see the vehicles in a dealer's inventory, with the X-plan price, but there will also be a link to the window sticker, so you can compare the MSRP to the X-plan price. They even have calculators that can take into account cash back incentives or special interest rates, and can figure monthly payments and so on.
I used X-plan last week for the purchase of an '07 Explorer E.B. I used the website to find the ones in which I was interested, printed the X-plan prices and the window stickers, and used that to figure out my "worst-case" deal before going to the dealership. For whatever it's worth, and assuming that the invoice price I was shown was the true invoice, the X-plan price for the vehicle I bought was $15 below invoice, if I'm remembering it right.