You aren't thinking very hard.
Camping - once you have or can borrow camping supplies - nearly free.
We are going to the lake in a few weeks - a friends six bedroom "cabin" - cost, gas and food.
We stayed in a friend's Villa on the ocean in Mexico a few years ago, cost - airfare (got a great deal), rental car, and food from the market (oh, and lots of rum from the market).
A few years ago we went to San Francisco with the kids - stayed in an inexpensive hotel, I went as part of a business trip, so car rental was taken care of - cost: cheap hotel, trolley and Alcatraz tickets, food, airfare. A friend gave us passes to the Monterey Bay aquarium.
I can book, today, an all inclusive in Jamaica for my family - with airfare for $186 a day per person. I can go cheaper if I want to stay in a worse hotel. That was without shopping around - its possible that Cozumel or the DR is cheaper.
For us, Disney is an expensive vacation - and we own DVC so much of the cost is already sunk.
Now, a lot of what we do is take advantage of opportunities - but that is what
free dining is. If you can go during free dining, that's sort of like having a friend with an oceanfront Villa in Mexico.