We're a family of 4 which includes a 14 yr old and a (at time of travel) 5 year old. We will be staying offsite in a rental home for around $800.00 for 7 nights. The home is about 10 minutes from the Disney parks off of 192. It has its own pool, 3 bedrooms, a pool table in the house, not the garage. We thought about staying on site but for the money, it just wasn't going to work. Last year, we found a mini-van at Thrifty for around $200.00 for 8 days. Then we got really lucky and Southwest ran a deal for $39 each way which was less than $100 pp. You do have to pay to park at WDW, but like someone else pointed out, there are a LOT of nice resteraunts off site to eat at. We usually eat a light breakfast (we're not breadfast people), split lunch at WDW, maybe chicken tenders and a sandwich. This is nice because you get to try more things off the menu.
We also save change, sold on ebay, alotted $$ weekly, etc. You'd be surprised what you can sell on ebay. My oldest DS made out nicely selling video games he no longer played, etc.
Once you go, you'll wonder how you ever managed not to!