Is it fair to blame authors for exorbitant book costs?
Short answer: both yes and no.

Partially, no they are not responsible, because some of it is a matter of costs and markets.  You're never going to see textbooks offered for the same price-per-page as romance novels, because the cost of creating and curating the textbook is so different.  So the base price of textbooks is expected to be significantly higher.

At the same time, however, authors have a lot of choice which publisher to work with, and some publishers are much more predatory in their pricing than others.  Most significantly, there are a number of university and society presses that can give a book quite good promotion and also a reasonable price.  Consider, for example, the MIT Press, and these widely used books: the CLR(S) Algorithms textbook ($70) and the SICP programming textbook ($49) and also free.  Moreover, academic publishing is a seller's market, unlike fiction publishing: in my experience, big academic publishers are always soliciting for authors to create books for them, and so there is space for an author to select (if not necessarily negotiate).

I would say, than, that any author who has courage of their convictions with regards to education can make a publishing choice that keeps their textbook well under $100 US.  Doing otherwise demonstrates either that the person is not caring about student costs or else that they think like an economist.