15,000 miles per year, 15mpg, .$20 more per gallon = $200 a year more AT MOST.
Now lets assume you lose 1 little mpg for going with the dirt. Without a doubt you will, if not immediately then over time.
At 15,000 miles per year, and a change grom 15 to 14 mpg, you would use 71 more gallons. At $2.20 a gallon, that's $156.20 in extra gas. So if you only loose 1mpg, you would save a whopping $43.80 over the course of a year.
Also consider if you put the engine under load, as in hauling or pulling steep grades your gonna loose more than 1mpg as the engine will want more bang from the gas but won't be able to get as much. It will have to work hard for longer because the ECU will be running in a watered down mode.
Our LX routinely gets 15-17mpg, with most of those being 17mpg. I have had to put regular in it before and when I hit the hills I knew it. I run the same stretch of interstate on cruise control at 68mph with the same 6 adults every weekend during football season. I get 16-17mpg and the vehicle pulls every hill without downshifting. With the regular gas, same people, cruise on 68mph, it downshifted (screaming) on at least a third of those hills, and my mpg was around 13mpg. All that for less than $50 savings annually? No thanks.
edit: with my mileage dropping that significantly, it actually cost me more money to run poorer!