The oil change is another $40, incidentals can happen when flying too. My in-laws had the door on the plane blow off, as they taxied down the runway, ended up with an additional night in Detroit, and a loss of a day of vacation ($115 plus meals). A co-worker had his flight home cancelled, because the company went bankrupt, ended up with an additional 3 days in Cocoa Beach and 2 lost work days ($450 for his hotel, and $700 in wages). A friend had his luggage lost, and spent $250 dollars on clothes for the 4 of them (he eventually got $100 back and a damaged suitcase with some damaged clothes- 1 month later).
Other factors have to be considered;
1. Are you staying onsite?
2. Do you mind being tied to Disney's bus schedule?
3. Do you plan on going shopping, or off Disney property?
4. Do you intend to buy souveners?
5. How long is your vacation?
If you intend to do anything off property, you will either pay obscene cab fees, or rent a car. However, if time restraints are an issue, the 3 and a half days of driving, can be a problem.
For us, the drive is part of the vacation, DW researches the hotel for the way down, and makes sure it has a nice indoor pool, and decent restaurants nearby. We have a DVD player, and new movies for the kids. I do all of the driving, and usually put at least 15 hours in on day 1, so that we are swimming at our resort around lunch day 2. The trip to WDW is the start of my vacation, so we have no time issues. We enjoy having our own vehicle, to go shopping around Orlando, and to come and go as we please. As I said earlier, we have driven the last three years, and have had few issues. In the future, we may fly, but it will be because of time issue, not money, because we have found driving much cheaper.
Our quote (cheapest we have yet) from Northwest is $294 each round trip from Detroit for a total of $1168 CAD. It departs at 6:40 am, so we would have to get a room the night before for $120. Add about $25 for gas and another $35 for meals and we are at $1328 to fly. I'll take the $550 (including oil change) to drive, and spend the $800 on a few extra days there.