This guy is a philosophy professor, so if he is assigning a book every 1-2 weeks, in a 14-week semester, let’s say that’s like 8 books.
Buying 8 books for under $100 is cheap. It does sound like he takes care to craft an affordable syllabus.
Of course, if you are taking 5 classes a semester it’s gonna add up, but this is really not on the egregious end of things.
And sure, there is more contemporary philosophy, and it's great he's keeping the books affordable. But if it's anything early 20th century or prior, don't be so surprised people are going to read what's in the public domain instead.
I mean, literally the same page — so if someone says “what did ya’ll think about this quote on p.156” I could actually get to it in time instead of scrambling to find the passage in whatever I printed out from Project Guttenberg.
For this reason my strategy to save book money was usually to get the assigned books from the library and camp out in front of the copy machine for an afternoon.