Hi Bill,
I think Greg touched on a lot of the reasons. Most photographers are looking for a train that is as authentic as we can make it in 2019. Many are also looking for specific, spectacular spots, such as Rock Tunnel. Folks who have been to that location know well that it won't hold 130 people.....or even 80. Thirty is more like it. Phantom is not quite so tight, but still won't take a big group. The Friends of the C&TS have recently offered trips for as many as 80 people, which avoided the tight spots and still offered a pretty authentic train. Heck there are a lot of places on the C&TS that can indeed accommodate 80 people. But they couldn't offer $100 fares. I'm afraid those are gone for good. Roughly $100 might still get you a ticket on the regular tourist train, but that's about it.
Charter costs have soared in recent years. Ten years ago, you could do a decently authentic charter for a group of 40 on many railroads for $200 per person, per day. Not anymore. Now, it is at least double that, and in some cases, a lot more. At least back in 2015, the C&TS was charging about $10,000 per locomotive per day. I suspect it is north of that now. Part of it is that the Part 230 rules governing steam locomotive maintenance didn't exist back in 1988. The maintenance cost per operating day is significantly higher now. Coal costs are higher too. And the real kicker is personnel cost. Crews can only be on duty for 12 hours. Running on a railroad like the C&TS, it is very easy for a charter to exceed the crew duty day, requiring a second crew. Ka-ching!
So yeah, the higher fares are related to things like authenticity, re-lettering, group size, space at the run-by spots, etc......but the real reason for the higher fares is because the railroad's costs are higher, and because they are determined to make money on these events.
/Kevin Madore