Whether or not you will save money depends greatly on how you look at it.
If you order moderate to expensive meals on the DDP, you will pay less by using the DDP than if you got the same meals, with all the desserts, and all the snacks, and paid for all that OOP.
However, many people find that if they don't have the DDP, they do not order desserts at every meal, do not go to as expensive a restaurant, order meals that do not cost as much, or get less snacks. As a result, they spend less on food than if they got the DDP.
WDWinfo has sample menus. You can use that to try and figure out what works best for you.