When we went (4 people) on the DDP we spent under $500 on top of the price of the trip (12 nights). Tons of food on the plan and only real cost was the souveniers. Don't forget to factor in ride photos if you tend to buy them, they can get pricey if you buy a several. So it really amounts to how much do you want to spend on souveniers? We loved having already paid for the trip and only needing to pay for anything extra we wanted.
We spent $500-$700 for 8 days and that's including the cost of stroller rental for 8 days. That was mostly for souvenirs. Didn't need money for food (on free DDP) and transportation (used Disney trans.).
Wow. We can actually get by on less than $100 a day counting food! But then I guess that's because my DBF and I are students and have to be careful with our money. I can't wait until I'm able to easily blow $100 or more a day!