Find out when the book you want was first published, approximately. Sometimes copies of books which have been reprinted many times are expensive in their early printings (near when they were first printed) and newer printings, but not those printed in between.
For example, Oz books were published near the turn of the 20th century. Early and First Editions can cost a LOT of money.
During the 1960s, Reilly & Lee reprinted most of the Oz books L. Frank Baum wrote, although those might be pricey, they shouldn’t cost as much as the first or early editions, in most cases.
Later, around 1980 or so, Del Rey reprinted the series yet again, as large format paperbacks, they likely won’t cost as much as the first or early printings either. But a new book is likely to be around $30, at a guess. Just to see if I’m right, I’ll look up a random Oz book.
Baum wrote 14, but at #6 he tried to end the series, if I remember correctly, so we’ll use that book as a test piece.
Baum, L. Frank – Emerald City of Oz, originally published 1910 by Reilly & Britton. There’s one reading copy (“It’s had a hard life.”) for $20, the rest start at $200 and go up.
The 1960s reprint (1964) seems to go for around $60, a good bit less than the average $200 price of the early printings, yes?
The DelRey paperback (1993) starts at $1 and goes up from there.
No, these are NOT the only editions of this book. There are ebooks, POD copies, etc. with prices as varied as you might imagine. My point in using the above book was that books and their prices fluctuate and vary, especially when it’s a book that’s frequently reprinted, as the Oz books are.
So, if you want to read all of them, any format is fine. If you decide you want a set of actual books, it can save you money if you know when the book was originally published, so that you can look for later, complete editions, like the 1960s reprints and the DelRey trade paperbacks.
Collecting first editions can be fun (and also expensive). If you just want to own the books, it can be done for a great deal less by buying reprints. If you’re a first edition collector? Well, buying reprints allows you to wait for that perfect copy, or the deal of a lifetime!