I must agree with PP with the current add on fees ($7.8 billion last year
http://www.northjersey.com/news/business/050310_Airlines_made_78_billion_from_fees_last_year.html) for a family it would almost always be far less expensive to drive then fly.
If you are
only comparing airfare to gas + hotel (if breaking the trip into two days) + meals on the road + wear and tear on the vehicle, then yes, driving appears to be cheaper.
However, if you assign a dollar value to the time spent travelling, flying could turn out to be cheaper, as it is in our case. Each day spent driving is a vacation day spent (in my case) and a day that goes unpaid (in DH's case since he doesn't receive paid vacation days).
If we were to only account for DH's loss of pay for just one day spent driving in each direction (total of two days), even though we'd most likely break the trip up into two days in each direction, his loss of pay would easily be equivalent to, or greater than, the airfare that we pay to fly.
If you assign a dollar value to my vacation time, then driving is that much more expensive for us. (A vacation day does have this value to me, since a vacation day I spend driving is a day that I can't stay home if DD is sick or I need to take care of personal business). But, let's assume that you don't want to assign a dollar value to vacation days...It's still cheaper for my family to fly because of DH's lost income (actual, real loss of pay).
Now, factor in rewards travel or frequent flyer miles, and flying gets to be even cheaper. In our case, four of us are flying free on reward travel to Orlando, and paying to fly home (I did it this way to keep us all on the same reservation in each direction). I didn't spend extra out of pocket to earn the rewards. Some of the rewards were accumulated miles from previous travel, even though we don't fly more than once or twice a year. The remainder were accumulated by charging our groceries, gas and other necessary expenses to a credit card affiliate with the airline's frequent flyer program and paying the balance in full just about every month (it's money we would have spent anyway on necessities).
So, I think comparing the price of driving to the price of flying is more complicated than just adding up gas + meals + hotel and saying that amount is less than airfare. JMO, YMMV.