For a work of fiction, maybe around $10 (in deadtree) and preferably less for an ebook (I have, in years past, bought new hardbacks, but that isn't something I'd do now-- except maybe for an anthology.) For non-fiction, it depends on how bad I want the book and how much I can afford to spend. I believe that the most I've ever paid for a book is under $100, though.
My non-fiction tastes lean toward science books that will either soon cost pennies for a ex-libris hardcopy (in barely touched condition) or else become a sought-after OOP rarity that sells for multiples of the original cover price (and unfortunately I can't predict which it will become.)
Just for fun, I checked to see what the most expensive book on my Amazon wish list is-- $192.99 new, $28.82 used. (At least a couple, I see, are not available at all new and are over $200 used. But on the other hand for example,
this book, filled with glossy color photos, many of them full page-- makes a good art and culture book even if you aren't interested in the science-- sells for as little as a measly penny. )