In 1976 I bought copies of Apostol's "Calculus" for $19 for each volume new at the Caltech bookstore. These were published by Wiley.
That was for the 2nd edition of Volume 1, which came out in 1967, and for the 2nd edition of Volume 2, which came out in 1969.
$19 in 1976 dollars is about $86 in 2019 dollars.
Apostol's "Calculus" is still available from Wiley, and is still used in a few schools for their proof-oriented calculus courses (Caltech, MIT, Stanford, for example).
Wiley wants $283.95 for the current edition of each volume.
Guess what edition they are up to now?
The answer is the 2nd edition, from 1967 for Volume 1 and 1969 for Volume 2. They haven't even given them a new cover design in the last 50 years as far as I can tell. The design on the new copies today are the same as on my 1976 copies. All they've changed there is lightening the shade of the background.
(NOTE: this means that there is no downside to buying the international paperback edition. Sometimes publishers keep the international edition a few years behind the American edition, so for frequently revised textbooks the international edition might not match your course. Not a problem with these books. The international editions can be found for around $20 per volume, including shipping. Abebooks is a good place for this).
So...what changed since the 1970s to explain the $283.95 Wiley wants for these books instead of the around $86 we might expect based on the 1976 price?
The other big name in proof-oriented introductions to calculus, Spivak's "Calculus" was $70 when I bought a copy of the third edition in 2004. That one has had one new edition since then, which is currently $100. $70 in 2004 would be about $95 in today's dollars which is close enough to $100 that I can believe that one's price is explained by inflation plus revision costs for a new edition.
But for Apostol, where it's all just reprints of 50 year old books, I can't see anything other than "because we can" to explain the price.