We have four people, too. One variable would be the type of car you'll be driving, and the type of gas mileage it gets. For every trip we've ever taken, we've estimated costs of driving vs. flying, and even from MA we've figured 10+ tanks of gas, and depending on gas prices and the type of vehicle, this can get quite pricey. And that's only taking account for mileage down and back, not off the beaten path or around WDW. Your drive is probably at least three hours longer than mine, too.
Our family would also spend a lot more than $50 on minimum two days worth of food each day (two meals plus snacks - sandwiches get old after a while), probably more like 3 days; and a hotel room for us, we estimated at around $120+ for somewhere like a Days Inn in a safe location near the highway for one night (on the way home, to break it up). Then add in tolls and such, and our estimates are up around $1200+. Not to mention feeling exhausted when you arrive and for a day or two afterward, my being a horrible back seat driver

and tired driving on unfamiliar roads. I'd rather pay airline prices, but search for the best deal available. I guess everyone's costs are different that way.