I would probably budget twice that for the average trip. Prices do vary some from state to state, but the bigger thing often is your cars gas mileage and the time of year. It would take a car getting at least 30 mpg and gas prices around $3 a gallon to see that low of a price.
That is not impossible by any means, but the average car isn't going to get that many mpg, and in the spring and summer prices tend to typically be up around the $4 mark.