The way I look at it, if you're driving in, have a larger party (such that you can't fit everyone into one standard Disney room), and come during a peak season (e.g., Christmas, Easter/spring break, summer, etc.), the more the economics of staying off-site pay off.
If, however, your'e flying in, have a smaller party (such that you can fit everyone into one standard Disney room), and come during a non-peak season, staying off-site is relatively much less attractive.
For example, last Feb. our family of four flew in, stayed at CSR, and had
free dining. The room (including tax) was about $190 per night. Food cost for the entire trip was about $120 in tips. We took Magical Express and didn't rent a car so, while in Orlando, our transportation was "free".
If we had stayed off-site, we'd have probably picked something like a Hampton Inn (IMO, comparable room quality to CSR). Assume we could get this for $120 a night (including tax) and that it included a free continental breakfast. So far, for a 7 day/6night stay, being off-site saves $420.
Let's then assume a rental car (with car seats) and gas costs $30 a day. Parking is $14 a day so this adds up to $280, dropping the off-site savings to $140.
Next, let's assume we buy one snack per person per day in the parks and eat dinners at chain restaurants out of the parks, averaging $15 a day for the snacks and $60 a day for dinners (including tax and tip). This adds up to $450, which is $330 more than staying on-site with free dining, making off-site $190 more expensive overall.
So by staying off-site in the above scenario, I'm paying $190 more for the privilege of:
being able to drive into the parks instead of relying on Disney buses;
having a "free" continental breakfast in our hotel; and
having access to a greater variety of off-site restaurants.
But I'm losing the privileges of:
not having to wait at baggage claim;
not having to wait at the rental car counter;
not having to mess with car seats;
not having to drive, look at maps, or worry about getting lost or getting into an accident;
not having to park, travel a long distance from my parked car into a park, and trying to remember where I parked at the end of the day;
not staying at a themed resort with beautifully landscaped grounds, a great pool, hot tub, and water slide;
not having access to EMH;
not having the convenience of eating meals in the parks;
not being able to check my bags (for my return flight) at the resort;
not having to return a rental car.
So, in the above, scenario it seems like a no-brainer to stay on site. But if we were going during peak season (with higher rack rates and no free dining), the calculus changes considerably. And if we were a larger party (such that we couldn't all fit in one standard Disney room), the calculus changes even more.