Just go to the wdw website and price out your exact dates, etc... and leave off the dining. That will be what you will pay on FD. You will be paying rack rate for the roomm, which is where the difference come in.
In our case, we have decided to stay club level at BC, which with the 40% off deluxe the PIN is the better deal by $250. If I would have stayed in a standard room (no club level) FD would have been the better deal saving me about $100 compared to the PIN.
I just played around on the wdw website to see the alternatives. But to get the FD rate, just leave off the the dining package. Also be sure you did not click though one of the other promotions (kids stay free, etc.) before pricing as that could give you incorrect information.