When that happens, the impact on students can last years. The bottom line. Since the time spent speaking could be spent writing, most writers have no compunction about charging. Well, almost no compunction. You do sort of feel like that sometime.
Some writers do, in fact, speak for free.
But others have made a career of author visits. Parents unhappy with the caliber of author visits to a local school might suggest that the school do a bit of research before booking. The structure of the in-school program also can determine its success or failure. An author who meets with individual classes will have far more impact than will the distant superstar who declaims to 400 fidgety kids in the auditorium. Then comes the autograph session.