Sorry I haven't read the rest of the replies - so I may be redundant.
DH and I can rack up $300 in meals at the park pretty quickly. And we don't drink at any park or order appetizers/desserts often. But we can spend $50 on a table service meal with sodas or $25 on counter service with little effort. That's just for two of us!
To counterbalance this, however sometimes we'll drive off property for lunch or dinner before heading back to the park (but we only do this on very long days 9am-midnight).
Otherwise we plan on one paid meal and one 'brown bag' meal. I'll load up our day bag with a few PB&banana sandwiches, raisins, oranges, trail mix, chips, hummus, string cheese... Whatever snacks are on sale!
And we carry water bottles & crystal light or propel packets (there are other options, kool aid, mio, etc) to mix in for flavor. We don't use 'nice' water bottles, just re-use bottled water for a day.
IMPORTANT! If you're taking kids (older than stroller age) get each a lightweight backpack and have them carry a few items each (especially snacks/water since the bag gets lighter as the day goes on) that way the parents aren't playing pack-mule all day