Don't forget to include the subjective value (up or down) of the plan itself after everything is done. For example, since I'm bad at saving money (I like my toys), the pre-paid and pre-budgeted nature of the plan works well for me. I typically place a "value" of about $50 overall on this, so even if I were behind by that much, I'd likely still pick it up as it's worth about that much to me to be able not to have to worry about the cash during the trip. This works inversely too, as if you feel that the limiting nature of DDP is something that's less than ideal, you may decide it's not worth it even if you're saving ~$50 or so.
Totally agree with this and just want to emphasize it more.
Many people claim they love the pre-paid nature of the plan -- Not having to "worry" about their budget once they arrive, knowing most of the food expenses are already paid. Some people claim that even if they aren't saving anything, even if they are spending more -- the peace of mind is worth it.
Some people claim there is a value of "not having to look at the prices" on the menu. Of being able to order the most expensive item, without second guessing their budget.
Conversely, some people feel the plan is too limiting -- You lose a lot of flexibility. If you decide your full and want to skip a meal, then all your savings may go out the window. If your 11 year-old eats children's meals, you can lose all the value. And some people feel the plan pressures them to order more expensive items than they actually desire, or pressures them to order desserts that they don't really want. (I may be in the mood for a salad for lunch.... but I know if I order that instead of the steak + ice cream, I'll lose money).
So really, there are some people who believe the plan is worthwhile, even if they are losing a few $$$. And conversely, there are some people who feel the plan is not worthwhile, even if saving a few $$$.
Personally, I find the plan worthwhile only if projected savings are 15% or more. Otherwise, I'd rather have the freedom to order what I want, when I want, without worry.