The MK's Capasity is 100,000 people. I dont think they hit that every day, so lets just say they get half that number on average. so that would be 50,000 people. Again lets go low, and say the average cost for a ticket is $50. That would be $2,500,000 per day.
The other parks get less people than MK does. So lets say they only get 33% of what the MK does. That comes to 33,000 people per day. again, we will use $50 per person. That comes to $1,650,000 per day for each other park.
So a daily gross for all 4 parks using the math above would be.
149,000 Guests
$7,450,000 per day from ticket sales
or
54,385,000 Guests per year
$2,719,250,000 per year.
That doesn't include the water parks, i couldnt find any real numbers other than the capacity at MK which is 100,000 people. It is also said the monorail carries 150,000 per day. So i went with a number that sounds about right. If you add in food, plus gifts, plus hotel stay, that number will probably double. I think the average person would spend roughly $30 at any of the parks for food / gifts. Then there is the hotel costs... and the water parks...
They make a pretty penny.