The cost of driving depends on if you just want to know how much cash it will cost you right now or if you factor in depreciation on your car. Most jobs that require driving factor in depreciation, but you may not want to count that now, since you won't have to pay the money for a while and because you'll be using the car for your own pleasure, not to do a job.
The main costs are going to be gas and oil change. The oil change is relatively cheap. If your car gets the oil changed every 3,000 miles, and if an oil change is about $30 where you live, that's $.01/mile. The cost of gas depends on the mileage your car gets. If you don't know it, factor it a few times when you're doing mainly highway driving. To get the price of gas, you could use the current average national price, or you could just guess. That's probably what I would do. I think gas is running about $1.22 around my house right now, so I'd probably multiply by about that much, since we usually have to fill up in Georgia, and their gas is usually a bit cheaper than ours.