We always drink lots of free ice water to keep hydrated, so no need for us to buy bottled water.
I just never have thought much about getting the most value out of snacks/meals on the DP. We like to get the standard DP (1 CS, 1 TS, 1 snack p/d p/p). This is always more than enough food. The value comes in everyone being able to eat whatever they want whenever they are hungry, without worrying about it. That's kind of a big deal when you have two hungry teens in the family. We don't live that way at home, but it sure is nice to relax and enjoy it that way at WDW.
Having said that, I do enjoy choosing some things on the upper end of the price scale especially when at Epcot during F&W.