[jacobolus]

Did you look at the picture? https://i.imgur.com/0qiTKSQ.jpg Books I commonly buy secondhand that look roughly like those pictured are anywhere from $10–$200 each (depending on how common the particular book/edition is); we’re talking about pretty ordinary old academic books, nothing fancy or extremely rare. A pallet of books is ~500–1000 books (there are maybe 400 in the picture, but the blog author claims that is a "sampling").

[TheCoelacanth]

That's selection bias because you are only looking at books that you actually wanted. An average book is worth much less than a book that someone actually wants.

[jacobolus]

An average (~worthless) book is something like a pulp romance novel, political book by a sitting politician, self-help guide, .... These are printed in the millions and used copies can typically be found for $1–$5 + shipping costs. That’s not the same kind of books primarily shown/described here.

Any scholarly person who loves old hardback books and spends a few decades collecting ones they personally want or need is going to end up with some worthless books, a large number that sell for $10–50 each, and a few that are worth hundreds each. It’s just inevitable, unless they go out of their way to only collect junk.

If I had to guess I’d put the price of the old man’s collection in the $10k–$30k range. But it’s plausible it could be more, if he collected anything rare.