This has been looked into by a lot of DIS'ers.
The consensus seems to be that for what you'll pay in base rental, milage, generator fees, and gas, you could buy plane tickets, get a rental car, and two rooms at a value or one room at a deluxe.
You're probably looking at $2000 for a nine day base rental with some miles, and another $200-400 in excess milage.
Gas will run you another $1000 or more.
Then you've got to pay campground fees. Expect minimum four days round trip travel time, so you'll need to overnight someplace on the way as well. At WDW plan on a minimum of $50 per night for your campground.
Anne