Terrific question, thanks for posting!
I'm hoping someone will post their trip report of what they they paid without dining per day for meals including where they ate.
I think it all depends on the ages in your group. For us, we are mom, dad,
DD-5, DD-9 so my children would not be sharing a meal. We are getting the dining plan for the first time. We think it's worth it (edit- for this trip, next time to WDW we'll be camping so no DDP) so we already have the meals paid for when we get there; we can have character meals; and not worry about looking at the menu from right to left.
Here's what I think we would spend without the dining plan in the parks. Very rough estimates since I didn't really check any prices at restaurants.
Without dining eating in parks
$5 each person for breakfast
$8 each person for lunch
Dinner is so flexible depending if you get counter service or table service so I'm just going to use a low figure of $8 per person
$10 each person for drinks/snacks throughout the day at parks...this might be flexible if Mom and Dad grub off the kids snacks
So my total without dining is $31 per person. The dining plan is $39 per adult, no travelling off-site, no worry about tips, character dining included so you don't have to run around the park looking for autograph opportunities, Mom & Dad get their own snack
Now if you go offsite, I'm sure you can save by getting pizza but I don't think you would choose that for every lunch or dinner.