Personally, I don't figure depreciation of the vehicle, because we drive our vehicles into the ground. My husband's car is currently worth about $400 - another thousand miles isn't going to make a different anymore.
And while some people don't appreciate the family time...I was raised on road trips and I think they're fun. We've already done two 10-day road trips with Russ, and one 4-day road trip. WITHOUT a dvd player! I remember reading in the way back of the family station wagon, because neither of my sisters could face backwards. And really - the time one of my sisters stabbed me with a straw in a McDonalds in Abilene is a way better story than the time one of my sisters puked orange juice on Dad's pants on a plane.
When driving, we also take food from home, so eating costs are down - and we do eat in the airport, which brings the price up.
Plus, you've got to figure what you're doing the extra day, given two days to drive vs. one to fly. We'd still probably take the same amount of vacation time, so an extra day means more $$ for tickets, meals, hotel in WDW, etc.
Plus, a 13 hour drive? And a hotel? Wimps! We'd drive it in one day.

Like Anne's husband, we've done 17 hours to Jersey, 11 hours from Mississippi, and 15 hours from San Fran to Washington.
Of course, this is all a moot point, since we live in Washington and flying is the only option. For WDW.