> If I offered you a BMW M5 for $2000, would your first thought be, "hmm, I should check to see if someone is selling the same car for $1800"?
Firstly, probably yes. If you're selling way below the market price, there's something wrong with your BMW. But leaving that aside...BMW M5s are not nearly not as easy to produce or copy as books. I could probably generate a (horrible) learn to program in C book in a matter of days and put it up for sale for $30. I would appreciate someone telling the world "Don't buy this, it's a bad idea."
Now I maybe you have already read the book and know it's not particularly bad, or you think Zed's reputation as an author is good, or something else like that – and it's fine to offer those as rationales for buying the book. But the argument that "Any book about C is likely to change your career* and you're quibbling over $30?" is a terrible argument IMO. The likely thing that happens with the average C book is that you read the first 10 pages, can't grasp some point that the author is particularly bad at explaining, and wander off to watch a Youtube video or something.
Now if your book is so engaging that it prevents this sort of attention wandering, that is a book where you can reliably present this argument. One example (from my perspective) are the Feynman Lectures on Physics. I can read them for ages and ages without getting bored. If you or anyone else in the thread knows about that kind of C book, let me know. K&R (surprisingly) is the closest I've come.