It's hard to answer this question because it will vary so wildly among people, depending on your habits. Even my own budgets have changed drastically over the years. When I was a kid my family saved up for years to afford just tickets and hotel, so we spent close to nothing else in the parks. We brought in our own food and water, ate meals in our hotel room, and never bought souvenirs. So I'd say our budget was $0-$30 a day.
As a young adult with a partner or friends, I couldn't afford much besides a couple of snacks and maybe one souvenir, $20 or less. So, budget was $10-$40 a day.
Keep in mind that you can bring food into the parks, as well as your own refillable water bottle. I highly suggest a hotel with both a fridge and microwave. This allows you to have hot meals when you go back to the room for a lunchtime break, or when returning to the room in the evening (it makes even more sense when you're going during the "off season" when park hours are shorter, because you can just skip eating an evening meal in the parks and have it in your room after close).
Now, as an adult that can afford a bit more, I still bring in my own food and water, but allow myself some meals in the parks. I typically have breakfast at the hotel, food that I've brought in for lunch with some snacks bought in the park, and counter or table service in the parks for dinner. I'm not big into souvenirs unless I find something particularly unique, but I could not see myself spending more than $30-$40 on an item per trip. So, in total, that would make my budget about $60 per day, including myself and my husband.
Basically, it's possible to spend practically nothing in the parks if you plan things strategically. Or it's possible to spend thousands of dollars.