Right on....My airfare this year from New york was $1100 for 4 people round trip. It would be AT LEAST that much to drive when you figure in gas, food, hotel, tolls and all that stuff.
You are probably correct but only if you are going to fly down and spend less time there then the total of travel time and touring time. For example if you fly down and spend 5 days at Disney then you have spent $1100.00 for airfare and whatever it cost you for lodging/food for the 1 day flight there and the one day flight back. Total of 7 days including travel.
If you drive, I would guess that the average person takes two days to travel there and travel back so there are 4 days on the road with hotels (one night each way) and food plus around $550.00 for gas (current calculation). Factor in the 5 days there which is the same expense as before and you have got approximately the same cost but it was spread out over 9 days (two more days).
I cant' speak for you because I don't know your individual habits, but all the people I know, that choose the option of flying will opt for the same number of days which would add two more days to onsite vacations time. Disney isn't going to give you that extra two days, so if you fly and use the same amount of time as if you drove, then you still have the cost of hotel and meals for the same number of days. If you add to your park time you also just added admission fees for two extra days and on top of that you spent $550.00 extra on transportation.
What that all boils down to is that it is very seldom cheaper to fly with a family then it is to drive. What you sacrifice is time at the parks, many people find that extra expense worth it because they like being in the parks and not so much being on a road trip. Those two reasons are the only things to consider, time on site and road trip time. It is almost never cheaper, but, it is a whole lot easier, if not a bunch less flexible, to fly then drive.
Now my brain hurts!