I'm a big fan of DxDP, but the additional cost of the plan ($44/adult in 2013) is just the start of the "extra costs".
For the OP, you're looking at 6 nights with 4 adults, but eating 7 days with 4 adults. The additional cost of DxDP would be $540. You'd have 72 table credits. Based upon your 2 meal day plus eating snacks, granola, etc... in your room for breakfast, and one signature meal, you're using 60 table credits (6 days * 2 meals * 4 people plus 1 day * 1 meal * 4 people plus 1 day * 2 TS credit meal * 4 people). With the additional 12 table credits, you could have 3 table service breakfasts for the group, or upgrade 3 dinners to signature for the group. Each TS meal comes with app, entree, and dessert, plus a soft drink. Except for alcoholic drinks and tips, you're set. Let's factor those in... $20 in alcoholic drinks per dinner is $140 for 7 days. Tips at 18% for 72 sit down meals is about $500.
Doing the comparison, you've got the DxDP upgrade, $540, plus the alcohol at $140, plus the tips at $500, and you're at $1180. Of course, if you did not eat all 72 meals as sit downs, you'd save on tips, perhaps significantly. Or, if you stayed at your current plan of using only 60 meal credits but used all of them as TS meals, you'd cut the tips by $100.
So, in this particular case, DxDP is about the same as your projected OOP extras on the regular DP and you'll get an extra snack a day (which could be used for a bagel, breakfast pastry, cupcake, etc... for breakfast) plus have the equivilent of 12 meals to spread around as upgrades. To me, that seems like a "no brainer"!