I think that site is great for understanding the configuration options for any specific model. However I highly doubt it shows the true dealer cost. The dealers also benefit from hold-back and other programs to help fund their profit. Invoice maybe, but probably not cost.
I prefer to use the site to get approximate MSRP / list price numbers and to model different discounts. From what I've read, a good discount is anything over 25% off MSRP. A great discount is closer to 35%.