I thought the hotel/air/car expense was the obvious biggie here, so I left those out 'cause I wanted to see if the ticket really was a bigger expense than the food or merch for most people. Based on early returns, it isn't.
Also, let's all remember that many people visit parks as a day trip, and the hotel and transportation expense isn't a big deal for them. But tickets, food and merchandise still can be.
This is quite the opposite of going to the movies, where the price of popcorn and a drink is often much higher than the ticket price.
Of course, it gets complicated when you add in dining plans and extra snacks (Butterbeer?)...
For me, food/drinks is a biggie. Especially at Epcot. The margaritas alone could rival the cost of airfare. Nevermind the meals, there are the in between snacks that will get you. Like when you are on your way to dinner in the world showcase and you need something to munch on during the walk and you end up buying a turkey leg. That adds up.
I always find it amazing to see what you spend on bottle water by the end of the week. Bottled.....water!!
Tickets to WDW have a sweet spot of $250 to $300 for a multiday pass. For a family of 4, it comes to $1000. That IS a lot of money. With food, there are many options especially if you decide to rent a car. You get access to the many off-site restaurants and the numerous supermarkets. At the supermarkets, I will buy the next day's breakfast, lunch, drinks, and snacks to bring into the park. With a stroller, it is much easier to pack all my stuff. Certainly, if my kid is older, the stroller goes so we'll all bring backpacks. Eating dinner at a theme park is an option since obviously I can't pack everything. Fast casual is usually the better choice. I will only go to a super deluxe restaurant on one special occasion.
As for souvenirs, I won't buy much. Much of the junk I bring home ends up in a box in the garage. From experience, souvenirs is a waste of money. I might buy an overpriced souvenir photo as a keepsake. However, my digital photos provide the majority of my memories. I take plenty of video so my trip is captured and I can go back and see it many times on the computer.