Since we've done both the driving and flying thing, there are two things regarding the cost of meals / food when driving . . .
1) Wherever you are - home, on the road, in the air, in WDW - you
need to eat! Eating = money.
Since we realized that very simple but often overlooked factor

its made a difference in our planning and don't put too much emphasis on that cost. IOW we're going to eat whatever we do, even if we're staying home
2) Driving does save us money as we can better choose what to eat and how much to spend if driving.
I pack picnic lunches and dinners, if driving through. We can take turns eating as often as we take turns driving. Or make an event of it by finding a cool little place to stop and picnic.
Then I always buy ahead (at great savings) and pack ALL (or most) snacks, water, juices and soda. None of the liquids can be bought ahead and taken on the plane, so that too, saves us some big bucks!
(also, while I definitely buy treats for the trip, I sure don't buy additional big ticket thiings that have been mentioned. If our kids don't have enough of those to take anyway - and they do - they're getting yet another trip to the world and THAT anticipation is already a big ticket treat! LOL )