I think you're really smart to ask this question here and on the Resorts board. Usually people ONLY ask over on the Resorts board and get the standard 24/7 Pixie Dust answer.
Even though we own at DVC, we have stayed off site many times. We usually stay in a regular hotel room and not a condo, but I really want to stay in one of those Windsor Hills townhouses with their own private pool. Maybe on our next trip over Spring Break while my DD is still in middle school and will enjoy it.
I think your monetary saving are:
* Offsite hotels and condos are almost always cheaper than onsite hotels.
* If you have hotel points, you can even stay for free (we stayed at the Hilton Bonnet Creek and the Hyatt recently for free).
* You are more likely to eat lunch or dinner offsite since it's on your way "home" instead of paying the very expensive Disney food prices.
* If you have a condo ...
** You can cook breakfast in your room at the very least.
** You have your own private laundry so you can carry fewer clothes with you, saving on luggage fees.
Your expenses are:
* A rental car is just about a requirement.
* Parking fees at the WDW parks (free if you have an AP).
I think that you will have to do like
cherylsmike and make up a spreadsheet. I did that for a family trip where we were considering having a 12 person family reunion at a pool home. In fairness, we ended up at Pop Century because
Free Dining was announced and that was back when you could get the regular
DDP included at the Value resorts. Pop was more expensive than just the pool home alone, but it turned out that we would have paid more to stay at the pool home and then pay for food in restaurants. We really never planned using the kitchen for dinners.