Authors do charge a lot of money for visits. A hell of a lot.
The last place I was at, the easily impressed psycho head booked somebody who had been on TV in a show she liked as a motivational speaker. We got an ex child star telling everybody that he made five grand a week and spent it all on Coke and now he lives with his Dad and then refused to sing a song because he'd just released a record. So what the kids were left with was 'go on TV, take drugs and you'll be a pop star'. Six and a half grand that cost. From somebody who had been in Hollyoaks ten years previously.
Then again, there was also the fundamentalist Christian preacher who told the entire school that they would go to Hell if they didn't cast out all gays and demand that they were taught Creationism in place of Science and the CofE were all Satanists. In a CofE school. £3.8k for that one.
I get emails coming through all the time giving us 'the opportunity to meet' somebody who is visiting schools in our area. The prices for absolute nobodies are in the mid thousands for a couple of hours. Oh, and they usually want somebody to provide professional sound equipment, operate it and everything for a film crew to record for their promotional material. Including catering for said film crew, as though it's impossible to pick up a bacon roll from the cafe on the way over in their van.
Christ knows what would have been charged for an actual famous person.