We always take snacks for the room and pool, and make a little snack bag for during the day. We each bring a bottle of water and refill with water along the way. Over the years I have collected

a few small plastic containers, just right for this, so that everything doesn't get smashed.
This is a sampling of what we might bring:
Granola/ snack mix, chips/pretzels/crackers, hard candy, pop tarts or breakfast bars, dry cereal, grapes or apples - ( nothing that will melt in the heat)
Gatorade packets for the water- we prefer Gatorade to PowerAde. Also when the kids when they were little we had a small soft-sided cooler that fit under the stroller, that I would bring in Capri-sun's, or juice boxes. I would use a freezy gel pack thingy, to help keep things cold.
As far a the cost of snacks they are in line with any other event type venue, in some cases less. Now this is my opinion, we went to Monday night football this week in Atlanta, and it was 41.00 dollars for 2 cheese burgers, fries & 2 cokes.
Also to me the snack portions at WDW are good size and you can share most items.
If you want to check out the price's head over to Allears.net they have the most up-to -date menus with price's.
Have a wonderful time.