It's easy, go to map quest and plug in your address and then destination WDW, this will give you a route and estimated miles. Now you have to know your cars MPG, lets say map quest give 1000 miles to WDW, your car gets 20 miles per gallon, divide the MPG into the miles to WDW and that's the amount of gas you'll need one way. Times that 50, above example, against the prices of gas, say 2.00 a gallon, and that comes to 100.00 for the gas. If you drive straight thru throw in a few bucks for tolls but if you stop over night, add the motel room rate plus what you figure you'll eat in route, unless you're packing a cooler and not stopping for food and add the whole deal up. That's a good estimate for a one way trip, times that by 2 for round trip. Pretty basic but it gives you a good guesstimate of driving.
I believe the going per diem rate, businesses give employees, is .24-.28 cents per mile. That accounts for gas and wear and tear on your auto if you drive for your job. So you can see, driving in the example can run you 100.00-240.00 one way, no stops or 200.00-480.00 round trip, throw in a motel down and one back @75.00 you're up to 350.00-630.00. Granted you can save because it's one vehicle and many people verus train or plane at no vehicle but per person tickets. At least it's a start.