It's a simple question with a wide variety of not-so-simple answers.
My general opinion of the dining plans is that they don't save money -- in almost any case, you can eat for less out-of-pocket, sharing meals, bringing in outside food, etc. So the question really isn't "how much do I save on the dining plan?" but rather "is the extra cost of the dining plan worth it for how it will affect my overall vacation?"
There is a big psychological difference between trying to economize by ordering "light" off the menus or passing on meals you might have otherwise wanted to do and having the comfort of knowing it's already been taken care of ahead of time and that you have the freedom of ordering off the higher end of the menu without breaking the budget.
Of course, a dining plan has its restrictions as well -- you're basically forced into eating in a certain fashion, and if that's not how you want to dine on vacation... well, then even if it were a net cost savings, would it really be worth it?
For any of the plans, it only makes sense if that's the way you want to eat (in terms of numbers and types of meals), you tend to order off the higher end of the menu, you want all of the entitlements (including dessert and, for DxDP, appetizers) at every meal (or most of them), and that you will use all of your credits.
Beyond that, I find the DxDP the most economical, because you can get your money's worth far more easily, without always having to order the top-end menu items. But that also presumes you're using the vast majority of your credits (3/4, at least) on table service.
DDP and QSDP tend to be "break even" plans for the most part, unless you really know how to work the system, and with the current rates, it would be easy to pay more for the plan than you would OOP for the exact same items.