chess coach $100?! really??

Hard to say without any info on the coach
I find 100 a tough amount, but 100 to me could mean alot more than 100 to someone else
It also depends on the engagement the coach has with the student. Are they regularly catching up/checking in? Are they reviewing games of the student whilst they sleep? Are they available to answer questions throughout the week? Are they preparing lessons for student in their own time? Or is the $100 solely for the exact 60 minutes then "see you next week"...
If you are paying for a coach who cares about your improvement, has the knowledge and resources to share. Then it may be worth it... again it all comes down to individuals

But outside of session time adds value. That is something to consider.
One session with a coach isn’t likely to make that big of a difference ( I could be wrong). So super high or even relatively high amounts seems hard to justify.

I don't think it's particularly unusual for people who charge for one hour for a specialist service and don't have a regular stream of clients to be charging that amount. To get a living wage from it isn't so easy.
Given that anyone, anywhere in the world who you can communicate well with can provide that service then I'm sure you could get a lot cheaper though.

Depends on the coach. There are a lot of high profile coaches whose ratings may not seem as impressive as you'd like (still at least an IM) but who have a proven track record of students who've made great improvements. They're free to charge what they like for their time, it is THEIR time after all. No one is forcing you to pay $100/hour, you can very easily find other coaches with more affordable rates.
Is it just that they have a title?
What would they have to do or provide that could make them worth the price?