Reddit reviews Electromagnetic Fields

It's publishers, not so much bookstores. Especially easy to see with advanced books/courses - not the intro calculus level ones, but specialized topics. For instance, as an electrical engineer, IEEE is a professional society that also helps publish a large number of the texts. Directly from Wiley-IEEE (again, the publisher) most of the books are still in the $150 range...often even after a 20-25% discount if you're an IEEE member.

And again, these are not the intro/100 level classes. Example. At least with the intro/100 level classes you can sometimes get a well-used, old edition, or international edition that's a lot cheaper...but this is a publisher thing to a decently large extent.