I get about 17mpg average in my 99TJ. It's been about the same since I bought it. I carry about a 100 lbs of gear and run 31x10.50 bfgoodrich ko's.
If you're on the bubble about whether to buy a jeep or not, do this. Compute what your other choice of vehicle will get in miles per gallon. Let's say for example it will get 23 or so mpg. Subtract say,17mpg of the jeep from the 23mpg of your other choice.In this case (Your Mileage Really May Vary haha.) Then figure the cost of losing that 6 mpg per tank every time you fill up. (based on a 20 gal tank )
I figure you'll be losing about $14.00 per tank based on $2.00 cost of a gallon of gas. My math ain't that great but I think I'm close. After you get a number you can work with, figure how many tanks you'll average per month/year.
If you still want a Jeep after all this, then figure you'll skip buying that suspension lift, or those new tires, or that Warn winch, or any of those other fun those accessories for a year or two until you've amortized the extra fuel expense. ;)