Two years ago (summer 2009) we had the DDP for 2 adults, 1 child, and we kept every receipt and added up exactly what we would have spent OOP for every meal vs. the dining plan cost. It was a substantial savings (almost $300). See below: (my last category below is OOP costs like tips and extras that we would have incurred with or without the DDP)
DDP COST: $545.82
WHAT WE ATE COST: $827.36
$64.58 3 TS breakfast at Tusker House
$8.00 3 snacks at Animal Kingdom
$89.76 3 TS dinner at Boma
$2.75 1 snack at Epcot (popsicle)
$4.57 1 snack at Epcot (Chocolate Pretzel at Kringle-La)
$50.68 3 CS meals at Sunshine Seasons
$92.34 3 TS dinner at Teppan Edo
$2.66 1 snack at Hurricane Hannas (fries)
$6.92 1 CS (child) meal at Hurricane Hannas
$27.97 2 CS meals at Beach Club Emporium
$39.00 2 CS meals at Wolfgang Pucks Café
$11.58 3 snacks (Dole Whip floats and citrus swirl) at Magic Kingdom (Aloha Isle)
$32.43 3 CS meals at Beach Club Emporium
$2.76 1 snack at Main Street Bakery (cinnamon roll)
$95.24 3 TS dinner at Crystal Palace
$59.28 3 CS meals at Pepper Market
$29.88 2 CS meals at Beach Club Emporium
$13.85 4 snacks at Goofys Candy Company
$13.13 1 CS meal (breakfast) at Gasparilla Grille
$14.84 1 CS meal (lunch) at Polynesian (Captain Cooks)
$3.08 1 snack (Dole Whip) at Polynesian (Captain Cooks)
$84.29 3 TS dinner at Cape May Café
$66.96 - 3 TS lunch at Animal Kingdom Kidani Village (Sanaa)
$10.81 3 snacks (Diet Coke, two zebra dome boxes) at AK Lodge (Mara)
TOTAL = $827.36
OOP Charges while on DDP:
$11.00 - Tip at Tusker House
$16.00 Tip at Boma
$10.11 Kring-la School Bread and Carlsberg Beer
$26.00 Refillable mugs
$8.00 waters/soda at gas station
$30.56 starters and tip at Teppan Edo
$8.47 breakfast items (cereal/bananas) at Beach Club Emporium
$17.39 tip at Crystal Palace
$2.12 cereal for Thursday breakfast (Adrian)
$7.00 tip at Pepper Market
$15.12 tip at Cape May Cafe
$1.06 banana at Beach Club Emporium
$4.25 Saag Paneer at Sanaa
$13.00 tip at Sanaa
TOTAL = $159.08
One factor here is that
DVC members don't have to pay an upcharge on the DDP during the more expensive times, which helps. But I think where you really get your values is on the dinner buffets and the more expensive CSs, and the child DDP is a real value if you are going to buffets. Some buffets, particularly in the more expensive time, cost more than the child's daily DDP charge.
One place where the math could break down is if you would never get snacks, desserts, etc. if you did not have a dining plan. If the dining plan does not really reflect generally how your family likes to eat then you are not saving anything, you are just prepaying for stuff you may not really want or need. But for us the DDP works. The one year that I did not get it (2008), I was horrified every time a bill came at one of the buffets or restaurants. Even if DDP was just break even, I love being able to order whatever we want and not think about the price while we are there. Somehow it is more palatable to me personally to pay upfront $700 or something, and then just not think about it again, rather than be faced with bill after bill of more than $100 per meal. I know that may not make sense financially, but I like the prepayment. The point about prepaying with a Disney gift card makes some sense, but it is still money you are calculating, so for me I think I would still be thinking - oh, let's not get dessert or a drink, too expensive, save the money on the card.