The best place to get info on the
Disney Dining Plan, oddly enough, is on the
Disney Dining Plan board in the restaurant section of the DIS.
As far as pricing and valuation of options, I think there are only two analytical techniques.
One is to compare the price of whatever DDP plan you chooose to the inflated menu prices of the Disney food outlets. (We find all of them to be about 25-40% overpriced.) Making that comparison is
a fool's bet because the DDP is obviously priced to win that comparison.
The other possible comparison is to compare the cost of DDP vs. what you would
actually spend paying cash. If you would not really eat dessert with each and every meal, cash may work out better. If you would not eat a sit-down dinner every single day (with dessert every time) cash might work out better.
Also, because of the pricing structure, the composition of your group matters greatly. If you are 2 adults, one 10 year-old and one 12 year-old, you're going to have a tough time justifying DDP. If you are one adult with 3 <10 kids, you'd be nuts not to get DDP.
If you have a car, eating offsite will almost certainly save significant money. However, if you are a captive onsite, and you
really would do a sit-down meal or character meal every day, DDP may save you some inflated dollars.
That's the money part, but cost/value is not the only legitimate reason for buying DDP. Some people simply like having their food expenses pretty much known prior to the trip and paid in advance. OTOH, other people don't like having their vacations revolve around dining arrangements. Those are personal considerations and nobody here knows your family as well as you do.
It's best to consider all of the angles
realistically, and make the best decision for your family. DDP does not work well for
our family (2A, 1 C), but it may be great for yours.