I often have professors who will ask in class during the first week or so whether anyone's had trouble finding the book and if they had the book in the bookstore.
Inevitably, they also ask the question, "How much did the book cost?"
Then, no matter what cost people respond with, they always act completely surprised. "Oh my goodness, I had no idea it would be so much."
Then comes the worst part: They apologize. "I'm so sorry, I didn't know it cost so much. You know, textbooks are such a racket, aren't they?"
Here's what I think about this.
If your use of the book really depends on how much it costs, you can look it up on Amazon before you assign it. Add about $10 or so for the bookstore's cost. If the price seems ridiculously high to you, it probably is.
Then after that little apology, they always say something like, "Oh well, it's a really good book, it's probably worth that much."
Don't apologize if you don't really care.
Students already assume that teachers don't care about how much money the students have to spend on textbooks. So don't feign surprise or act like you're sorry, because we already know you're not.