I bought it from an upperclassman for $5 or so back in 1975.
It's pretty much the same story for the other college textbooks of mine.
When mine wore out [1], I replaced it with the international edition. Exact same text, but paperback instead of hardback. Around $30 with shipping. Amusingly, I could either have bought from a dealer in India for around $10, with $20 shipping, or from a dealer in the US for around $25 with $5 shipping.
Good sources for international editions of textbooks are abebooks.com an biblio.com.
[1] since I don't actually use calculus much, I tend to forget it. Every few years I get tired of living with the shame, and re-read Apostol (although once I did Spivak for variety).
Math is something I'm surprised there's so many editions about. I don't think Calclus 1 has really changed much, so why do we need 10 editions for it? I really do think it's just for the sake of the publisher.
But I do share your questioning the need to constantly write new Calculus textbooks. I often read about complaints about public schools being stuck with "old" textbooks, as if grade school math, science, and history have somehow changed in the last few decades.
When the recession hit a lot of students asked teachers whether the earlier edition texts were still okay for class. Most of my teachers completely understood the situation students are in financially, one teacher went on a tirade against the school's bookstore.
I only graduated back in 2011. During sophomore year I found all the international edition textbooks on ebay. US Retail $100+ would often be ~$40 itnernational. Only thing different would that it was in black and white vs color.
Average semester cost of books usually ~$200, if I had paid full price it could easily been $500-$600. The bookstores only give 20-30% back most of the time and heaven forbid if a new edition comes out and the price drops 90% on resell.