Instead of mileage (which can vary as most have said depending on a lot of factors), you might also think in terms of fuel cost. If I was planning I would assume thirty to forty cents a mile for gasoline to determine what my trip costs will be. Of course, gas prices go up and down and vary greatly around the US. $2.05 down the street from my house today, I get about 9 miles to gallon so that is 23 cents a mile. I would figure a lot more than that for the variables over the next several years of travel. I suspect a cost of nearer thirty cents than the top of 40 cents would be appropriate, but I remember a few years ago when 40 would have been way too low.
So, take a guess and figure out how much a dream trip will cost at say 35 cents a mile. Just one way to think about this subject.
Paul