In the Gas Prices thread Janice B said it cost them $235 US plus $55 CDN to drive from Waterloo to Orlando and back on the I75 route in a Montana mini van. They returned on 8/6.
We drove this past Feb, cost was $300 US by minivan. Quite the savings compared to 2004 when we took our motorhome and the cost was $600 US. This is driving from Essex, Ont.
Try this link, it is only good within the States, so start with whatever city you are crossing at. You can judge how much gas you will use to the border.
Our trip from London in May was 175 US total for the trip in our Montana we were getting about 600K out of half a tank of gas we only ever drove 65MPH though.
You have to consider the mileage of your vehicle. The Montana and Venture both use the same 3.4L engine and use 8L for every 100km driven on the highway. Buffalo-Orlando is in the ballpark of 1900km so even at $1/litre (which I seriously doublt you'll be paying that much once you pass Pittsburgh), you're around $150 each way.
To get your vehicle's gas mileage, go to the manufacturer's web site and you'll find it somewhere. If you drive a Hummer... I know a good broker who can give you a deal on a second mortgage!!