If you are looking for comparable (apples to apples) accommodations outside of Disney vs. on Disney property, then even with the cost of a rental car, you won't save money by staying onsite.
We had a two bedroom, two bathroom villa for the last week of August at SVR. Total cost was $120 per night. Our 8-night total was $960.
Same time frame....a similar unit at Disney (two bed/two bath) at Saratoga Springs has a rack rate, with tax, of $695 per night.
http://www.mousesavers.com/2014-saratoga-springs-room-rates-season-dates/#rates
Let's say I luck in on a 35% off deal. That brings the cost down to $486.50 per night, or $3892 for the same 8 nights.
That leaves us $2932 ahead. The rental vehicle was $290 taxes in (and that was a Jeep Grand Cherokee.....a full size car was half that). Gas? Let's say $100, just to be on the high side (we didn't even use $100, and that was with a round trip to Cocoa Beach and some other side trips, but its an easy number to use). Then let's deduct another $120 for parking ($15 a day x 8 days). No additional cost to get to and from the airport, as the rental car will do that nicely.
We're still
$2422 ahead. If my numbers are off anywhere, feel free to correct me.
The only way you can get cheaper is to downgrade all the way down to a value level resort at 260 square feet. Then you can't compare a the two bedroom villa to the value level room, because its not an equal comparison. In that case, you should start comparing the price of the Disney value to, say......the Best Western Lake Buena Vista (definitely nothing more elaborate).
Even saying "well, staying offsite you can't get the dining plan, and the dining plan saves us money!", you could break it down like this.
For our family of 3, over 8 days, an offsite stay is $302.75 cheaper PER NIGHT even after the rental vehicle, parking and gas are all calculated inn. That's $100 per person, per day. I can't imagine the dining plan would save us $100 per person each and every day of our stay.
And remember....all these figures are assuming a 35% discount. That's a lofty assumption!