My current NORMAL service area is anywhere within 30 minutes drive time from my office. If the location is well within the green area below, there will be no extra charge for driving. Along the borders of the area, and beyond, I base extra drive time charges (currently $1 per additional minute, one way only) on the estimated drive time given by Google Maps.
Example: Google Maps tells me that the drive time from my office to the intersection of Highway 75 and Bethany Drive is 33 minutes. I might tack $3 on to the price for a shoot at that location.